The amount of spells you can scribe depends on your INT.
According to the manual: At an INT of 19 there is no limit to how many spells you may scribe At an INT of 18 you can scribe 18 spells per level At an INT of 17 you can scribe 14 spells per level At an INT of 15/16 you can scribe 11 spells per level At an INT of 13/14 you can scribe 9 spells per level At an INT of 10-12 you can scribe 7 spells per level At an INT of 9 you can scribe 6 spells per level With INT below 9 you cannot scribe spells at all.
Generally all you need to scribe spells beyond this limit is a Potion of Genius. Once scribed the spells stay in your book forever. In BG2 scribing spells gives (spell level x 1000) XP so it pays off to scribe worthless spells. (Of course ToB's delete option gives way to lots of cheese when you have money to spare.) But in BG1, an INT of 15 is more than enough to scribe all the generally or occasionally useful spells.
--- Mating with duck is like vomiting in a too small bucket. Listen to my music: http://www.newgrounds.com/audio/view.php?id=907376
Oh that's the BG2 manual... though I don't think it differs much if at all. --- Mating with duck is like vomiting in a too small bucket. Listen to my music: http://www.newgrounds.com/audio/view.php?id=907376
There are usually only five or six good spells per level anyway, so writing dozens of spells per level is really quite unnecessary (unless you like the cheese of free experience for doing nothing but scribing scrolls).
Aside from the rule mechanics for INT vs. max spells, there's also an interface problem in that even though you can scribe "unlimited" spells into your spellbook, you can only access the first like 12 spells (IIRC) scribed per spell level because anything more than that is written "off the page" and can't be accessed with the game's interface because you can't scroll down to it. At least that's how it was in BG1, not sure about BG2.
As others have said, since there's only so many useful spells per spell level, this is really only a problem for people going for "completeness" and being totally over-anal about it. --- Are you threatening me, master Jedi?