You actually have to take into account a bunch of different things. I don't grow pumpkins personally but we had them in the garden when I was a kid and with a good grow we could get about 12 but it required that my family really keep up on the nutrients, watering, things like that. They weren't huge pumpkins or anything but we considered it a nice little harvest! Here's a lot more information than I ever needed to know about pumpkins but you might find it helpful. :)

I believe that I got 6 or 7 from each of the two vines I had last year. They were the Small Sugar variety.

I usually get a half dozen or so of the smaller varieties. The larger Halloween types, usually only 2 per vine. The giant varieties usually only one, 2 if I am lucky, but then they are usually not as giant.