Are you using VoIP or an Analog system?
VoIP typically can handle up to 20, 40, may be even a 100 calls on a single machine assuming you have enough 'horsepower' and network bandwidth.
Analog systems on the other hand are limited to the number of lines you can connect to the Voice board. I believe there is a physical limit of 64 lines and this will require the Enterprise license last time I checked.