Mounds should be 4-6' apart.
More Info: It is important that pumpkins are given sufficient room to develop. Pumpkins that are planted too close together will yield fewer and smaller fruits and are more susceptible to problems such as disease and rot.
Give Them Plenty of Space
Plant 4-6 seeds per mounds that are spaced 4-6' apart. The soil should be at least 60-65°F before you plant as pumpkin seeds will not germinate in the cold.
Once the seedlings emerge and have two leaves, choose the healthiest two in each mound and discard the rest.
Interesting Pumpkin Spacing Study
It is widely accepted that planting pumpkins too closely will result in smaller fruits and poorer yields.
To demonstrate that plant spacing influences the size of pumpkins, Elizabeth T. Maynard of the Northwest Commercial Horticulture Program at Purdue University North Central Westville, conducted a study by planting a Jack-o-Lantern cultivar at two width intervals , one wide and one narrow, and a second giant pumpkin cultivar at the same intervals.
The Jack-o-lantern variety that was spaced the widest produced twice as many pumpkins per plant as those set at closer intervals and weighed on average 25% more.