i don't think there is a limit but usually try 30-50 people instead of going to big .... you could locate them through friends of friends or fan pages or game rooms
Iv had many accounts disabled for adding 99 daily. So id cut that in half-ish and go with around 40 a day.
I dont think there is a limit to it. Facebook would block your account if they would start reporting you abuse. So as far as I know, there is no limit to ad friends.