In GP4 the number of teams and drivers is fixed: 11 resp. 22 with two drivers per team.
In CSM mods you have the possibility to include a team selection and thus get more than 11 teams (and more than 22 drivers) on the whole, but still only 11 resp. 22 in each selection session.
Examples for this are 1990th mods (1992, 1994, 1995, 1996).
However, less than 11 teams is possible by creating fitting performance file(s). See 2005 CSM mod by tony and friends as an example.