I think that eight guests is a lot.

I would not accept the claim that since she has already told eight girls, you have to invite eight or feelings will be hurt.

She should not have told anyone, much less eight girls, before consulting with you. This is an important lesson for her. You are quite unwise to allow her to force you to do something you don't want to do on the theory that otherwise feelings will be hurt.

What if you have two sleepovers, 4 guests at each?