you are thinking of channels
if you have a tdm400 with 2fxo and 2fxs, then you can connect up to 2 phone lines and two analog extensions. If you have a single T1 port, that gets you 24 channels of whatever.
However the max number of concurrant channels is only limited by 1. limited destination channels for the calls to go to, or 2. what the machine can handle. IE, with the TDM card you could have two lines, and two analog phones, and 200 IP phones connecting to a VoIP trunk account for example.
put another way, asterisk can deal with all kinds of channels including VoIP, analog, and digital. Calls can be sent into or out of any one of them. The only limit is when you run out of channels to send the calls to.