Depend on the cultivar. 20 plants from 25 seeds is reasonable. Big pumpkins two to a plant is reasonable. Smaller ones will set more. They do need space, all bets are off when they are crowded.

Thank you! Ok, couple more questions. Can I order seed packs any time of the year and do they keep well, if I ordered now, they'd be fine next spring? Also, if I tried to keep seeds from my pumpkins now, would they be really variable in what they become because I had a variety of pumpkins (Big Max, Cinderella, Jack-O-lantern)? My understanding is they'd probably cross into a mixed pumpkin.

Throw out the seeds you have now as you will have no idea what they would grow into. Better yet, roast them and eat them.
You can order seeds right now and they will grow next year. But alot of vendors will be out of stock. There's no hurry. Wait until the 2010 seeds are out. Although if there are some good sales to be had you might pick some up at your local garden centers and whatnot. Sometimes you can pick them up for next to nothing.