leagaly with out liscense, in the USA... the answer is 2
27mhz and 49mhz are leagal in the usa...
the rest.. well cross your fingers you dont get caught..
So in the end...
27mhz
35mhz
40mhz
49mhz
57mhz
Using those might allow 5 running at once...
the 45mhz.. will likely conflict with 40-49 so dont use it.
asuming u can actualy find a 40mhz bit Ch.. I want one my self so if some one kowswere to get a 40mhz let me know, even if its a 4$ clone
.