Yes there are exceptions so it all depends on the variety you are growing.

In addition, even with the varieties that are self-pollinating having more than one plant increases the odds/percentage of pollination that occurs.

Dave

You don't necessarily need two of any cucurbit to get pollination; as long as the pollen is transferred from the male flower to the female flower, they will self pollinate. Breeders & seed savers often "self" cucurbits (pollinate with a flower from the same plant) to get pure seed.

However, I agree with Dave. The female flower is only open for a day; if no males are open on the same plant, no pollination will take place. More plants means more flowers, which increases the odds. More plants also increases the bee activity, which again betters the chances of good pollination.

Yup, more flowers open at any given time = better chances for pollination and fruit set.

If you have any doubt or want to take matters into your own hands, watch for a female flower to appear and then find a male flower thats open at the same time. Pick the male flower off, take the petals off, and brush the female flower down with the male flower's bits.

That'll almost always get the job done.

I agree.