Answer: So much depends on your soil, the weather, pollinator activity, variety, and maintenance, that it is impossible to give a truly accurate answer. However, according to the "Pumpkin and Winter Squash" growing guide from North Willamette Research and Extension Center at Oregon State University: [http://www.orst.edu/dept/NWREC/pumpkin.html]
"A common spacing of 3' x 5' would produce 2,900 plants/acre and up to 4,000 10 to 15-lb pumpkins, for a yield of about 20-30 tons/acre."
This refers to plants in rows, rather than hills, with rows spaced 5' apart, and plants spaced 3' apart within the rows. In my experience you get at least 2 pie pumkins per plant, so I suppose you might plant on a quarter acre with 500 plants as a minimum, adding a certain percentage in to take into account plants that fail to produce. But you may want to check with your state's Extension Service Fruit and Vegetable specialist before tackling such a large planting. I'm only throwing these numbers out as guidelines.
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