I'd never go above 4 per ip. Theoretically there's no limit, just think about hundreds of accounts on the same ip for people in the same business building, or students in a dorm. The problem is the usage pattern, if you use all of them to promote the same site/offers/links they will catch you fast. If you use them to do different things, promote different stuff you could be safer with a lot more on the same ip.
How do they track usage pattern? they have algorithm that do that?