How is \"single-user\" done?

[Ron Natalie]

The number-of-user restriction means "n users logged in at a time", you
are free to have as many accounts in /etc/passwd as can reasonably
be supported.  The system is usually restricted to one user by bugging
init to only start up only as many getty's as required.

Any reasonable UNIX hacker can defeat the number of users with a little
thought, although a trend is made on IBM PC's to omit the general terminal
case and write software that only deals with the console.

-Ron