The answers depend on your pumpkin growing goals. If you are aiming to grow a few super sized pumpkins then start pruning as soon as you see baby pumpkins forming. Just leave one or two pumpkins to grow. You should also thin out the young plants immediately  as less competition (less crowding)  means bigger pumpkins. If you want lots of pumpkins and don't care about the size then wait to prune and don't worry so much about thinning out the extra plants. Happy Halloween and Keep Growing, Marianne Binetti