Depends. Definitely 1, but you might get away with 2 depending on how well they grow. But you probably won't get many more melons if you grow 2, just less (or smaller) melons from each plant.

I'd go with 1. Plant 2 and cull the weaker plant.

Al

This is probably heresy, but I grew five sugar babies one year and five tiger babies in each of the past two years in a 25-gallon whiskey barrel. In each case, I got two melons per plant on three of them and one per plant on two of them (8 total). I would think you could grow two in 10 gallons. I would start with three or four and only cull all but two when the first fruit starts to form. There can be so many problems when they are young.