In this mode, the server maintains 16 players by adding robots. If more human players join, robots leave to make room.
See the file INSTALL.Newbie in the ServerVanilla distribution. It shows how to install and set up the server for Newbie mode.
Once set up, the mode can be temporarily turned off by changing etc/sysdef NEWBIE=1 to NEWBIE=0. The change takes effect immediately.