The minimum number of drivers for an official race will be determined by the
Competition Board and will set to 6 unless otherwise noted. Some Series will require at
least 8. The maximum number of drivers will be determined by the FIRST
Competition Board in order to ensure safe and competitive racing within each
Refer to section 5 of the Sporting Code for further details.
You can also find this information in the season schedule.
From the UI click Help and Support on the left, then click Race Schedule and for each series you can see how many (minimum) drivers to make a race official: