4 layers of paper
Answer from Piñata Boy
It’s common for the balloon to shrink a little when the first layer of papier mâché is drying — as the papier mâché dries, the evaporation can cool and shrink the balloon. The balloon should return to its normal size once it’s dry. If it doesn’t that means the balloon itself is leaking air. When this happens you can sometimes carefully pop and remove the balloon, then insert an uninflated balloon into the papier mâché shell and blow up the balloon to the size of the papier mâché. I’ve done that a few times, usually because I popped the balloon too soon, but sometimes because the balloon was leaky or I tied a bad knot.
If the balloon doesn’t return to its normal size once the papier mâché is dry and if you can’t inflate a new one inside the papier mâché, then you’ve just got to start over. I’ve had to do that a lot, too. I like working with balloons, but their tendency to shrink as they dry (or to expand in the Texas heat and tear the papier mâché) is one downside to working with them.