A jump rope.
or
Twenty. One to hold the bulb, two to turn the ladder, and 17 to be on the guest list.
or
The question is irrelevant, since you couldn't find the dolls even if you knew how many. (Note: Well, this was a good joke in 1983-84. . . .)
or
Bike helmets don't fit hippos
or
It ran out of juice!
or