ISBoxer will have no problem controlling that many toons, although you will probably not really use FTL. You should still be able to use all of the features, but if you're thinking about controlling 20 on the same PC you're definitely going to have performance issues. To run 20 on the same PC you'd probably want them all low resolution (turn instant swap off) and it might help to have a dual i7 or dual opteron (8-12 cores each!). I haven't tried launching that many in a while so I'm not really sure how well it would perform.

Whoever asked if the bosses are going to be easier in AV if it's 25-man, they don't really need to be easier... 1 healer 1 tank and 3 dps can easily take down the boss at 80 with 2 towers still up, I know this because I did it with my paladins a while back: (screenshot) http://isboxer.com/forum/viewtopic.php?f=10&t=292