20 spell slots can be pretty easy to handle in the low levels (1-10), if you get a new spell from the appropriate level every new level. Say at level 5 you get 1 level 1 spell, 1 level 2 spell, 1 level 3 spell, 1 level 4 spell and finally 1 level 5 spell. High levels could prove to be a little bit more work to micromanage, but vanilla 3E gets hairy in the high levels too, maybe even more so.If you're going to have that many spell levels, you might as well just have spell points instead. 4e had spell levels that were the same as the level you got them, but even that system didn't have 20 or 30 levels of spells, since you only got a new spell every couple levels or so and dropped lower levels ones as you went up in level. That, and you didn't have to worry about preparing most of them or deciding which spell should go in which slot for greatest effect. Having to micromanage spell preparation when spells can be prepared in different levels and having 20 levels of spells to work with would be a nightmare, IMO.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.