Q.I have A very big pumpkin plant– very healthy with lots of blooms, but only 1 pumpkin. should I cut off some of vines? must be nice 5 stems and
At least 8 ft long.

Certified GKH Gardening Expert
You can. As long as you are careful, clean, and follow the proper process. This article will help: https://www.gardeningknowhow.com/edible/vegetables/pumpkin/pumpkin-vine-pruning.htm
I would recommend testing your soil. You may have too much nitrogen or not enough potassium and phosphorus in your soil. These articles will help with this: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/extension-search
This article will help you to grow pumpkins: https://www.gardeningknowhow.com/edible/vegetables/pumpkin/pumpkin-growing-tips-for-halloween-pumpkins.htm