Ok , for the Saga answer , almost all Magi (99.99999....%) will have a magical focus.
It would be highly exceptional for anyone not to have one.
Mythic Blood carries an "automatic" Minor Focus in an area related to your ancestry. Thus, it is incompatible with the Minor Focus in Certamen that all the Tremere have.
Ah true. 
Another reason to ditch the tremere focus, then
Cheers,
Xavi
Uncommon but existing. Where "majority" of magi has a focus, common.
A bit of both. More unique Focii than not though.
My answer is probably correct for either PC or NPC. Roughly at least, i didnt count exactly as i dont have character notes to do so.
wow, your saga has at least 10,000,000 magi?!
Not quite. 
I don't know how to do .9 repeat (9 with a dot above it).
His point is that to have that many 9's repeating you need a very large number of magi. If you have even 10,000 magi and only a single magus does not have a focus, then 99.99% of the magi have one. To write any more 9's you need more magi. Since any given Order (varying by saga) is unlikely to have more than about 2000 or so magi, we shouldn't see anything beyond about 99.95% without hitting 100% exactly.
Chrsi
+1 Confindence Point to Chris
+1 xp in Reputation: Mathhole for gerg
(99.9 repeater)% is 99% + the sum to infinity of (.9+.09+.009+.0009+.00009+...)%
100% 
Well, if we can have 1.4 kids per family why can't saga have 99.999(repeating)% foci?
And even if it cannot, 99.999(repeating)% is exactly 100%, mathematically speaking.
OTOH, it must stop repeating somewhere, since IMS there's no one with Magical Focus.
I like a famous physicist's response to this (actually about unending division, but the same idea) at a talk: "This is the real world."
But your point about 99.9(repeating)%=100% was the point. That's not "almost" everyone, that is everyone.
As far as 1.4 kids per family, that's 7/5 kids per family, or 7 kids per 5 families. You only need five families to pull that off, not some ridiculously large number. Of course, I have no idea how to count conjoined twins in the Medieval mentality, but they probably didn't survive very often.
Anyway, back to the topic, I have some questions for those with sagas with nearly 100% of magi with a focus:
Do you have the magical foci develop as a result of training almost all the time? Or are they mostly inherent? If they're inherent, how do you deal with the Tremere?
Chris
Aptitude and mentality
some degree of training but the universal tremere focus s the result of inductrinaton
Both. Some have a focus inherently from before they can cast spells or in any way use it. Others develop it through "focusing" their training, their spells or somehow their mindset.
This is also part of why we´re ok with more than a single focus(never had one with more than 3(IIRC), but too many would probably be very bad even if 2-3 isnt a problem).
They are inerent. The classical boy that gets annoyed and the soldier anoying him suddenly dies of a heart attack or decides to jump out of a window while crying for help. Some might get narrowed or broadened during apprenticeship (from wolves to canines, or from metals to iron) but the base material is there. This is why magic is an art: everybody manifests it differently, even if in broad terms everybody talks the same.
Tremeres get a puissant social skill of their choice.
Cheers,
Xavi
IMS , i decided that all foci would be inherent , in potentia.
Opening the Arts , would reveal a focus , but not normally choose its sphere of influence.
Tremere training would template a potentially different focus and it then becomes the House focus.
Magi without an apparent focus , who gain one from Mystery Initiation ,
would simply be a case of a "recessive" focus requiring stronger impetus to form.
It's interesting to note that most of the responses so far for those playing in sagas with most/all magi having foci have altered canon in some way to make things work better. This isn't a statement against that, just pointing it out. It brings me back to my thoughts about House Tremere and the like.
Chris
To be honest it feels like the Tremere players are whining (though not necessarily unmerited).
Yes, I agree that a focus in certamen is one of the less useful ones, epecially as it undermines the validity of certamen somewhat.
However once HoH: TL became available, membership in House Tremere became a huge benefit in itself!
Yes, you're expected to work for them, but the House makes huge amounts of resources available, and while the same can be said of your covenant, it usually has more limited resources and demand more work (even if it's not very honest and open about it!).
This is the primary benefit of membership in the House Tremere!
It's not their fault if you fail to take advantage of it.
Ofcourse, far too many of us are stuck with the image of the evil bogeymen of previous editions and many SGs feel that it's wrong to favour one player over others with regards to access to resources.
Wake up! Smell the soy-kaff! These resources are the strength of the House. To choke them off is to deny the character his/her native advantage.
And even worse it is also to limit the stories available to be told.
We are fairly heavy on the foci and use the focus rules as written.
for my character, I don't often think to use foci, I more often use affinity with an art