As many phone lines as your business needs.
Typical small offices probably don’t have a CAS T1 (24 lines) or PRI T1 (23 media lines + 1 signalling channel) and that’s fine. The system can be configured to use just a few regular telephone company lines, through a channel bank or similar adapter hardware (ATA). As your company grows, your hardware can grow with you because the system is expandable.
One fully-loaded system can handle 8 PRI T1s (192 lines.) Bigger than that ? Make it two systems (a good idea for hot standby backups.) Or switch from traditional PSTN lines to a new (and increasingly telco preferred) VoiP solution, such as SIP trunking – your new call capacity might only be limited by available bandwidth and CPU restrictions.
UPDATES circa 2018…
8-port T1 cards are on the market!