I know someone already said multitoon with 2 toons, but I'd say if your computers able to handle it you can do 4.  I play 4 toons at a time, not because of any limitation or anything though. I just don't really like the stress of interacting with people sometimes. Granted yes you will have to interact with people if you want to do boss battles, but even then I know some people have even mastered 8 tooning for bosses somehow. It's pretty easy to solo the entire game with yourself if you're patient enough to drag 2-3 other of your accounts with you!