Four
Twenty-four lines, shared
Twenty-four people can be connected at once, in total, worldwide, and the three dial-in numbers all draw on the same twenty-four. That is the whole capacity. So: no unattended automated traffic, no crawlers, no scripted downloading, and please hang up when you have finished rather than holding a line open overnight.
Outbound mail submission, open-proxy ports and the usual amplification ports are blocked at the firewall. That is not a judgement about you; it is what stops one compromised machine turning twenty-four dial-up lines into somebody else's problem.