In some circumstances the ACC server will limit the number of available car slots on your server, ignoring whatever value you have set in the event/server configuration.
The server will always indicate why it has carried out a limitation like this in the logs (available on the server -> logs page of the Manager), so you can check why it is happening there if you are not sure.
Here is a list of limitations that we are currently aware of as well as the associated log message:
maxCarSlots 32 exceed the global maximum of 30 cars for public MP, reducing car slots to 30
maxCarSlots 30 exceeds the allowed slots by Rating requirement (10). Add Track Medal and Safety Rating Requirements to gain up to 30 slots (3 TM + 70 SA)