Covenants (p.90) introduces commentaries as a new type of book, similar to a tractatus be related to a specific summa.
My question is, how many commentaries can a magus write?
Possibilities are:
I would allow one commentary per book. As a worthwhile commentary, you've said all you're going to say in response to the book. Your opinion the subject may change later as you learn more, but I would treat any additional works as tractati with "background flavor." I also generally rule that would have to "finish" a summa before you can write a commentary about it. (Nothing in the rules about that, IIRC, just my sense of how it should be.)
I don't believe there's a limit in RAW.
But if a player wanted to write more than one commentary to a single summa, I'm fairly sure he'd soon be introduce to the Hermetic Limit of being eaten by a dragon. 
Mind you, we typically haven't used the rules for commentaries - some of us remember them from the days of the 4th edition and wish we'd seen the last of them then.
Since "Commentaries are written in the same way as tractatus" we assume each commentary you write is one less tractatus you can write from your "pool" of Art/5 or Ability/2. A commentary is effectively just a tractatus, with a boon (the +1 quality bonus) and a hook (the reader must have read the relevant summa).
After some discussion, we decided to allow multiple commentaries on a single summa by the same author (as per the rules as written). After all, if you can write a commentary on that Quality 21, level 6 summa on Ignem that will be gulped down in a single season, it makes sense that you can write at least two or three on that level 20, quality 10 summa that takes 21 seasons to digest completely. We did tinker with the idea of placing limits based on the level and/or quality of the summa, but in the end we decided they were just cumbersome and would not significantly limit abuse.
Yet, we've never seen a PC write multiple commentaries on a single summa in game. Putting all your tractatus pool into a single basket is ... risky, as an author who wants recognition or significant profit from his books. I would point out, however, that in our sagas books PCs study are typically better than those most PCs could write, and are thus mostly from authors outside the PC's covenant and acquired through trade. This significantly impacts choices authors make. I'd expect that if the majority of tractatus studied by PCs came from other PCs it would be different.
That´s VERY far fetched.
Why? That was my first assumption of what the authors intended.
I'd handle it the same way. Otherwise it would be a way to basically double the number of tractati you can write on a given subject.
Why? As I originally posted, I completely fail to read that into the text. I read "commentaries are written in the same way as tractatus" to mean they take the same amount of time to write and their quality is calculated in (almost) the same way. If commentaries were simply another word for tractatus with a little background flavor thrown in, I would expect that text to say that.
They are not tractatus with "a little background flavor". They are tractatus with a "Boon" (+1 to Quality) and a "Hook" (the reader must have read the corresponding Summa) which I think nicely balance out. While I readily agree that the text is not as explicit as it could have been, I think it's a reasonable interpretation (of a somewhat vague wording) that preserves the limit on the amount of knowledge a single author can pour out on a given subject.