The # of maximum vehicles is set to 90. The default is 80. It used to be set to 100, but at any one time 10% of the vehicles were sitting in spawn locations. Also, many vehicles just seemed to be salvaged for parts.
90 may be a good number, but it may go back to 80.
Sometimes there may not be 90 since vehicles get destroyed and may not immediately respawn on restart. Vehicle respawn is random based on chance and total vehicles in class (helis, boats, cars, military, civilian, etc), maximum vehicles on the server and number of available spawn locations.