Your question is about PBX! It has some trunks and some extensions (ex. : 4/16 or 8/120..)
The line is PSTN connection to this PBX. Is’s must be connected to Trunk. If we talk about analog lines – for 4 trunks possible connect only 4 PSTN lines.
In case digital – 1 T1 connection(trunk in PBX) is connect 24 PSTN lines.
For the VoIP GWs are same way – numbers of line = max # concurrent calls.
Use different codec will give you more or less used bandwidth… for the same number of calls…