Limiting logons to licensed number: how?

[J. Bradley]

As a purely academic question: how do outfits like
DEC (Micro-Vax II) and VenturCom (Venix, DEC Pro/Venix)
limit the number of "init"'s for logons?  Both companies
want to charge a user license fee by the logon.  Both seem
to ignore the fact that often one will want more potentially
active ports than the cpu and disk would ever support, merely
to facilitate incoming modem communication at random times.

Again as a purely academic question: is the trick done in
the kernel, "/unix", or in "init", or where?

J. Bradley