All your machines in the network regarless of how many show as one IP only that your external IP...
If you set up and allow 100 machines using your internet via router or wireless access point Virgin Media dont have a clew how many machines are using that connection, However the more there are the less bandwith there will be available to you and it will slowdown greatly any internet navigation.
All these 100 machines will show as they are 1 machine from the outside of your network.
Also if anyones pulls a stunt over your network like any illegal stuffs than it will be your account on the line and Virgin Media will cut you off if they see it happen or cop on....