Why you need to know that? if you set a fixed number of bandwith per client in profiles, you don't need to worry if they share the connection. Alone they could make it by means of a pppoe router (as adsl router without modem) or sharing with two nics in windows xp.
sorry, it`s seems questionable about 20 connections from single IP...
maybe.... connections behind ConnectionShare must have different TLL ? (if advanced user use NO mangle and make no changes with TLL)
this topic is very interesting.... new ideas wanted (and examples too)....