It totally depends. Do not underestimate the pumpkins.. I planted them last year and 2 plants took over a 10x15 section of the garden, and then grew over and through the fence. It was a mess. Cucumbers and cantaloupe (muskmelons) do not get too big, you could plant maybe 4 of each in a 4x8 bed if they have room to vine out in different directions. Summer squash is a bush plant not a vine (typically) and I would personally put no more than 6 plants in a 4x8 bed. Winter squash depends, some are bush and some are vine. Winter squash needs more sun than summer, for sweeter fruit, so I would keep them from being crowded.

Thanks for the reply. I did, after posting, do a little more reading on the summer squash and realized those were bush plants. I'm trying those in 7gal containers outside the bed, leaving the bed only for vining plants that I can train up a trellis. I settled on a smaller number of plants, but it does still include two large pumpkins and another winter squash. From what I've read, I can prune the growing tip off of the vines if they get too long and the result might be smaller pumpkins, but that's fine with me -- I'm not trying to win any competitions. I've also got a watermelon and two muskmelons in there. Just because I had extra seedlings, I added a tomato plant, a bell pepper, and some herbs, all of which I'm willing to sacrifice if things get too crowded. I may even put in some bush beans, since this should grow quickly while everything else is still getting going, and I can probably harvest them before it's too crowded. I guess I'll see how it goes. I'm considering this bed an experiment, and if the entire bed fails, I'll have learned something.