Callen, you have inconsistent assumptions regarding the lifespans of magi. Either they live and are writing for 90 years or they are replaced every 50 years. Otherwise some of the good quality authors from 1220 are the same magi from 1170.
Assuming exponential growth, you can model the total number of magi from a starting number. Lets start with 100 magi at the start of the order, 1200 in 1220 and a 500 year span. You can find the rate of exponential growth by solving 100*(x)^500=1200, which gives you an x of around 1.004982. Thus, the number of magi in any particular year is 100*(1.004982)^t. If you assume 1% of the magi can write sound tractatus and those are able write one every other year, you can integrate between 0 and 500 and get 1107 tractatus written total. Lets assume that 62 of them are cult lore or for some mystery cult or something specific like that. That leaves us 1045 tractatus. Lets assume Magic Theory is twice as likely to be written about than any thing else and all the other public mystical abilities combined are as likely as Magic Theory, and divide by 19. That leaves us 55 tractatus in any particular art and 110 in Magic Theory. Then apply that some are written in Greek rather than Latin in the Thebes Tribunal, and that some are lost and burned and or written by Diedre or fried by an overeager Flambeau or probably 40 left intact in the order. Then, you figure that one half are written in so distant a tribunal that it never reached your tribunal and you got 20 different tractatus left.
I admit, having 20 tractatus existing on every particular Art seems high. On the other hand, It doesn’t take much to drop the numbers further. Just drop the percentage of magi who are good authors down to .5%, and you are down to 10 books per tribunal. You can also assume a smaller initial size to the order, (maybe 80) and much more destruction either in the Schism War or when House Tytalus was decimated by diabolism. If your covenants are not the sharing type, tractatus that are owned by a particular covenant might not be readable, even though all of their members have read the tractatus does not mean they are willing to trade, since it would make it unavailable for apprentices and new members.
Oh, for people suggesting language in tractatus would change…Isn’t the point of Latin that it was not evolving like all the living languages were?