Oops! Umm I don't remember if its necessary. I have 2 and they're relatively small guys. The ones I have seem happy together though. They do separate for periods of time and do their own thing. From my experience, I would say do 2.
And actually, live aquaria is saying 30gallon minimum but I've heard 10gallon minimum. I could be wrong though. Maybe someone with a bit more experience will comment?
10 Gallon Community: 1 Betta, 2 Otos, 4 Neon Tetra, 1 Ghost Shrimp, 1 Nerite Black Racer
2 Gallon: RCS Breeder
In Progress: 20 Long Sulawesi Shrimp and Snail tank