I grew 2 minnesota midget muskmelon last year in a rubbermaid 20 gallon container. Got about 4 melons off of each plant. I would give each plant at least a 5 gallon container and definitely train them up a trellis for better air flow. Make sure they are not crowded and look out for signs of powdery mildew. I sprayed daily with a mild vinegar solution which seemed to stave off PM until the very end of summer.

1 per 5 gallon container is good. I just like to have 2 in case something happens to the 1. You can always cull later. Last year we had a hail storm, and it severed lots of my plants. Plants know when they are crowded and will produce less.