Well... actually yes but this is not logic.
The problem is that you have 1 public IP, which is fine, but your clients are behind a NAT. Now, if you want to enable RDP for each pc, you must modify every single port and... REMEMBER IT!
So, once you open RDP you must type: MyPublicIpAddress:PORT and the value PORT will be different for each host. If you have a network firewall, this is painful, because you must open EVERY port.
Anyway, to modify the RDP port do the following:
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp\ select PortNumber and edit, select DECIMAL as value, and add your custom port.
If you have a firewall, remember that you need to enable this port.