They really should be in a larger group than what I have. At least 3. But as Olympia said, they have a fairly low bioload, hence why I firmly believe that 4-6 could easily go in a 10 gallon with a betta with enough plants. Sand is def preferred, though I have sand and mine never bury, preferring to simply hide behind a rock I have. The inch per gallon rule doesn't really apply to them because they are shaped so different than a normal fish. When it comes down to it they are probably more like neons. Without a betta, 8-10 would probably happily live in a planted 10 gallon. Though like I said before, they are very sensitive to water quality. So don't go with more than 4-5 if you don't heavily plant or don't feel experienced enough with keeping good water. They are also fairly nocturnal. Sometimes they are out during the day, but they are most active at night so keep that in mind if you want a fish you see all the time.
Also, these fish can live 5-8+ years, and are thus a much longer commitment than a betta. I got mine as an impulse buy about a year and a half ago. They were only a dollar apiece, and it was definitely stupid on my part. I knew they didn't get very big, but I should have done more research on their other needs. They do seem very happy though.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?