How big of a pumpkin will you need for one pie?
A very small one. If you're including the peel in your puree, there will be almost no waste from your pumpkin- only the stem, seeds, and stringy center will be taken out. (Remember to rinse, salt, and roast those seeds!) You'll get almost two cups of puree for each pound in your pumpkin, when prepared this way. Two cups will make one 9" pumpkin pie. That means a 5-pound pumpkin will yield enough puree for FIVE pies. And if you use your 25-lb jack-o-lantern?...... Luckily, pumpkin puree freezes well! My favorite way to preserve it now is by making pumpkin powder. Less space, no freezer burn, and no electricity required once it's dry.
If you're using big pumpkins, you can still end up with dense, smooth puree if you know how to handle it. Clean, chop, and steam, boil, or microwave the pumpkin until tender, then run it through a blender or food processor, adding water only if it won't blend without it. You could use a potato masher, but it won't get rid of the strings. When it's smooth, you can either use it as-is (which is thinner than usual), or let it sit in a colander lined with cheesecloth or paper towels. Let it drain at least 1/2 hour. The water drained off has vitamins in it; use that in pancakes, smoothies, or whatever else occurs to you. Any winter squash can be used in pumpkin recipes. I've eaten "pumpkin" pies made of kabocha squash, spaghetti squash, and Hubbard squash. The spaghetti squash was a lot more watery (drain that liquid off!), and if only mashed, not pureed, the texture resembles coconut cream pie (in fact, there's a recipe for Mock Coconut Cream Pie that takes advantage of this!) Hubbard and kobocha are both dense, creamy, and sweet, and only required the 1/2 cup of sugar that I use with sugar pie pumpkins.