If you are doing video/voice i would say that 8 or 10 per radio? Where 8 in 2.4ghz and 10 in 5ghz? :)
If you just doing normal things like browsing and that kind of stuff i would say like 15 or 17 per radio?
Like Collin said it depends on what kind of application you using... if you got many internal application heavy ones you would not want many clients on one band... specially in 2.4ghz which there is a lot of interference and noise...
Rember also you going with old g which is just 54mbs...
Hope this info helps you
Cheers
Carlos