While I understand that you want to design your network so that tyou'll never get even near that 255 clients per radio, you'lle always have customers that ask the question anyway.
So just so I can give the correct answer (while convincing them of the silliness of the question).. what would happen?
Is this a hard limit, the AP running out of registers or something. Or is that simply a limit the technical marketeers imagined to stop clients from actualy trying to load their APs with such numbers?