I am hoping 50%...but DS went to another party a couple of months ago and he was the only child from his nursery who went (despite loads having been invited).
Ds will be 3 and I reckon 10 would be a nice number. Not all will be from nursery, but the majority will.
The thing is I don;t really "know" the parents to ask in person as we all drop off & pick up at different times...I would get more of a feeling if I knew them, I think.
But what if all 20 said yes