A good rule of thumb is 3 feet in all directions for each male betta. You should probably use young males that you raise yourself and as soon as you can sex them (before violence begins) move them to the larger tank.
Who am I? Well I am a Junior Member of the International Betta Congress and a breeder of white HMPKs, red dragon HMPKs, black/red dragon HMPKs, metallic multi HMPKs, blue masked HMPKs, and now multi color HMs. Soon to add black HMs. I will be showing in New Breeder class in fall of 2012.