1.) 5 I believe.
2.) Just one at a time. With the exception of some games, like LBP, that you can have 2 ID's on at the same time. I only know of LBP that does that..
Like I said, LBP is the only game I know that you can have two ID's online. But it's not like you can check your buddies on both ID's though. Only the main account.
you understand? lol I confused myself a little.
Ok, let's say you have your account and you sign in to that one. You power on another controller for your brother. It asks them to sign on with their ID. You can check your PSN friends, messages etc. on your account, but not your brother's. Get it?
You're not really signing in when you play multiplayer on LBP, you are just allowing that player to play on his own account so he can accumulate the trophies, reward etc etc that you obtain.