For penetration purposes, using an AC doesn't give you any penetration bonus if you have a score of 0 in the Penetration Ability so there it doesn't matter if you can't use any AC in that case, but one should still be able to use Range: Arcane Connection even with a Penetration score of 0.
That's not true about penetration purposes for two reasons. First, having an AC allows you to use Sympathetic Connections, further boosting your modifier. So using an AC with a Penetration of 0 give you a modifier of (1 + 0 + Sympathetic Connection bonuses), which can be greater than 1. Second, the Penetration option from Spell Mastery increases your multiplier in the same way.
Yes, you should be able to use Range: AC, and that is a very good reason to have a minimum of one unless rewriting it just to account for that, or just disallowing using Range: AC without penetration. Might as well put the minimum at one to make things work out nicely. On a related note, a minimum of one does still make things like Range: AC to teleport to new spot with an AC impossible without a higher score in Penetration, even though you're not necessarily really doing any actual penetration with either AC.
Multiplier can be increased, but 0 times BigNum is still 0.
Penetration Bonus = Penetration Ability Score x (Penetration multiplier). Multiplier starts at 1 but can be increased if you have an AC.
Penetration Spell mastery option does not add to the multiplier, but to your Penetration Ability score - which is a good point anyway.
Oh, whoops, I really multi-botched that one!
Note that in a number of spells you might, in fact, need two ACs even without any need to penetrate. For example to move a target at AC range to a place to which you have an arcane connection.
I think a slightly better solution than having the minimum is to say something like "In a single casting, you can use up to your Penetration score in Arcane Connections to boost your Penetration; note that Arcane Connections used for other purposes do not count against this total."
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I like this proposal. I was trying to find a nice phrasing for this issue. I don't have now 
I don't think one needs to fix what isn't broken. Is there any real need to cape the maximum number of ACs to a penetration value?
How would this interact with magical items such as The Horn of Champions from HoH:S p.13? The item relies on the Pen score of the magus activating it? A grog would need to learn Penetration to activate an item using multiple ACs, just as he currently needs Finesse to operate some items?
Would hedge traditions also be limited to 1 AC per score in Penetration? What impacts would this have on the traditions specified across the books?
(I'm sure if I rack my brain I can come up with other things that might need to be double checked if such a change is made.)
Usage of multiple ACs really does not seem like a problem anyone is experiencing (in my view at least), so I'd rather you don't fix it.
As for the initial post, yes, I think that's how it works in regards to magi A, B, C, D and the Target categories. An erratum is not completely necessary IMO, but it might be useful.
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I will drop the qualification, because otherwise someone will read this as meaning that, if I use an AC both for Range and to boost Penetration, it does not count agains the limit of ACs I can use to boost Penetration.
It does not use ACs to boost Penetration, so it would not interact with it at all.
This does raise another tricky issue, though, which needs its own thread.
Oh, actually, I was correct. I just botched the explanation. Penetration bonus is:
(Penetration + Mastery if Penetration chosen) x (1 + AC bonus + Sympathetic bonuses)
So even if Penetration is 0, Mastery can change the first part. So 0 Penetration score doesn't have to mean 0 bonus from using an AC. Of course, David is dropping the qualification now, so that doesn't matter here so much.
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Your first proposal does not make the distinction about ACs used for penetration boost and ACs used for other purposes at all, so if you want to impose a limit only on ACs used to boost penetration something on the text needs to change.
However, isn't the purpose to keep things mostly coherent? If so, why apply an arbitrary limit to only one aspect of ACs?
You are absolutely right; it's obvious even to me after re-reading it. How about adding solely?
In any case, my main point was not about the wording, but about the actual rule: to exclude from the "up to penetration" limit those arcane connections that are not used for penetration.
That said, I have to say that I tend to agree with Rafael Bessoni, in that I do not see that much value in limiting the number of "penetration ACs". Sure, the "big bundle of ACs for all sorts of useful targets" may seem an issue, but remember that "Arcane Connections must be stored carefully, or else they become links to different people or places (ArM5, p.84)". Keeping them in a big bundle seems a sure recipe to ruin them.
If I had to have a new rule about ACs, I'd much rather have "you can fix up to Magic Theory of them per season, and/or fix a single one as a 7-day distraction" 
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If the ACs have been fixed they should not risk being ruined. Making a big bundle of them will be a lot easier if you can fix several per season.
If we are now going to allow using multiple ACs at once for penetration - which up until this thread I had thought was not possible according to the rules - I would want to see a limit on the number that can be used, precisely to make sure you can't use a "big bundle of ACs"
Well, the text on p.84 makes no exception for "permanent" ACs.
And the text on p.94 about "fixing" just says fixing makes any AC into a "permanent" AC; it does not confer any other bonus.
So it seems that even permanent and/or fixed ACs must still " be stored carefully, or else they become links to different people or places", eliminating the "big bundle" problem ... if the troupe does consider it a problem.
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While I think this is interesting, this limits a few uses of fixed ACs (eg. using them on enchanted devices that you intend to carry with you). And a fixed AC is likely to be "stored carefully" at one's lab. Wouldn't it then become a connection to the lab itself in due time?
Again, is this an actual issue? Has this ever happened? Does it have a real chance of happening? And if so, what would be the actual impacts? You mentioned before:
Would you not need to be aware of the target and then, by definition, know which AC to use? The only corner case I can think of is the one where you are seeking a cabal of Tytali who all disguise themselves when outside, so that you will need to grab all the ACs you have managed to gather (AND preferably fix) every time you go out.
Honestly, if you managed to do that I'm not even mad if you use all the ACs at once. You earned it. It's your win. Congratulations!
(But I suggest you start a formal Wizards War before, just to avoid legal trouble later.)
It has not been even a potential issue until now, because until this thread I would have said the rules prohibited using more than one AC at a time, and that you could not have different Penetration Totals against different targets.
I don't think that "stored carefully" means "held untouched by human hands in the caveau of a Swiss bank". Keeping it in some sort of non-mystical container, possibly one that gets changed every now and then, should suffice. Ultimately, it seems to me that the text just says "you can't be too casual with ACs, show them the proper respect", disqualifying abuses such as AC-bundles (assuming they are abuses, which depends heavily on the troupe).
How often would I need to change containers? Once a month? Once a year? What if I enter a 7-year twilight? Did I just lost all of my fixed ACs? What is there to stop me from occasionally taking my ACs out on specially prepared containers, changing them as soon as I return to my lab?
I find the concept of semi-permanent ACs neat, but I think it opens a few cans that are not easy to close (hey, if we manage to do it, great!).
I'll take the chance to mention that from my first reading of corebook I took that fixed meant fixed. You can't take it out of the lab. Of course, we do have canon examples in contrary now... But it would also solve the issue.
But this all hinges on we needing to make "bundles of fixed ACs" impossible. Personally, I'm not convinced we need to.
First of all I think the issue with storing them carefully would affect other logistics than whether or not they could be used at once (at minimum you could still hold one in each hand). However while retrieving and using one AC might be trivial retrieving a set of them, each individually stored, is something I would expect to take some time.
My reading of the contamination of AC's is that if not stored properly they might become an AC to multiple targets, and the magus would be unable to decide which target it would be used for, which might not be a problem if it is only being used for penetration.
If people want to talk about storing ACs, could you do it in a different thread?
We have two opinions on this thread:
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The rules currently allow you to use an unlimited number of ACs to boost Penetration, and this is fine.
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The rules currently allow you to use only one AC to boost Penetration, and if this changes we definitely need a limit.
A clarification seems to be needed, if we want everyone to play the same way. The majority opinion seems to be that you could always use multiple ACs with one spell. That is certainly what I thought, so I am inclined to clarify in that direction.
I am concerned about the possibility of a ReCo AC Hoover spell to get Penetration boosts against all your targets, which is why I initially suggested the limit. (If you can't just hoover up everything, you need to target hairs from your target, which you probably can't do without magic that Penetrates their Resistance in order to check which hairs are theirs.) I could be persuaded that a limit is unnecessary, and would complicate the game more than it is already.
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Ah! Thanks for pointing it out; it had not occurred to me. The "AC Hoover" is indeed a problem. However, even limiting the number of ACs usable, it seems to still be a problem if you are trying to hoover ACs to only one or two targets, as long as you can hoover an area where they are the only targets with MR (which I think is rather common).
So perhaps the solution does not lie in limiting the number of ACs, but in limiting how "casually" one can harvest ACs?
Which is, in fact, something of a broader problem: if it's so easy to shed and harvest ACs, wouldn't most entities with MR take elaborate precautions? From my gaming experience, this has created tensions in PC behaviour between a) what game rules encourage, and b) what feels mythically appropriate.