One plant with two incomplete flowers: pistillate or “female” flowers and staminate or “male” flowers define a monoecious plant. (See this post about complete, or perfect, flowers.) I recently figured out that the reason that I wasn’t getting any fruit on my squash plant (Yes, singular---I have one plant growing in a large pot on my patio.) was that the flowers were typically only opening one at a time, lasting only a few hours, and that they were all female. Finally, on Monday, there were two flowers, one of each "sex", and lots of busy insects. Now I have one squash (heirloom scallop, Cucurbita pepo, from Botanical Interests, Inc). Whoo-hoo!
Although this sounds like pretty basic stuff (and me with a degree in horticulture, even!), it’s exactly the kind of thing you learn with experience or via a mentor. Nowhere on the otherwise informative seed packet did it say, “For best yields plant a minimum of three.” Or, “Single plants will have extremely low yields.” Or, “Not recommended for container gardening.” Corn is an example of a well known monoecious plant that always has planting instructions that include block or mass planting for improved pollentation.
Lesson learned? Plant more squash plants if you want a crop!