One giant pumpkin plant will produce on average 3-5 pumpkins, smaller varieties can produce up to around 12. There may be more female fruit then this on the plant, but not all of them will develop.
Most growers will have one pumpkin per plant
The general rules for growing giant pumpkins is that you will eventually only have one pumpkin on the vine, and you will remove the rest. This allows the plant to direct all the growth into the one pumpkin. Although there is some thought now it doesn’t matter how many are on the vine.
Some people keep a second pumpkin on the vine as a type of insurance, just in case something happens to the other one.
I’ve always had multiple plants growing all with one pumpkin on each plant. It is a system that works well and if you have the room something I suggest on doing.