I recommend half the number of guests for how old your child is. For instance, if your daughter is eight, invite four friends.
This also depends on how much space you have. If the sleepover is taking place in your child’s room and there is only enough floor space for two guests, don’t invite six.
The fewer people, the easier. But this might change what games you can play as some — like Pass the Parcel — work better with bigger groups.