Make sure the Server Max vehicles are like 15 and players is like 5. You can do it in IGSO, Or Prefs.
What i said, try that.
I think these are the right prefs.
$Pref::Server::MaxPhysVehicles_PerPlayer = "50";
$Pref::Server::MaxPhysVehicles_Total = "50";
Type these in your console, or use IGSO.
You can also change them in Advanced Config before you start the server.