1 maybe 2. If you grow 2, they will compete for space and be smaller.

To my understanding true potting soil is a space medium through which plant roots spread and water and nutrients spread and flow very slowly through to provide a three dimensional root and hydroponics water solution to the roots that may only have to get recharged once a day. Try this, 6 plants to one bucket placed on a pedestal and a 2 .5 ft radius around for the beans to spread. The beans will grow up then outward to get the space they need. They will start to hang down over the edge of the bucket and reach out even farther. As long as the roots get the water and nutrients they need, sunlight and space become the limiting factor. I over plant my spaces the same way and it works. Experiment with one bucket first to perfect your system.

Good luck

Sounds interesting. I have six cantelope plants that I need to grow in containers. Any suggestions?

Plants have two different types of root systems, Those that look like upside down trees and those that spread way beyond the plant like a grape vine. beans, pepper, eggplants, onion, beets, broccoli,and potatoes all have contained root systems. squash and melons sweetpotato have roots that go all over the place and probably won't grow well. The plants will grow well at first then shut down a good bit. The smaller the fruit the more likely you'll do ok. On the other hand if you have very light weight soil, it will act as a 3 dimensional hydroponic system but you will have to provide a flow of water. Give it a try, but don't bank on it and don't let more than one mellon grow at a time system. I

Good luck