[REQUEST] Vehicle respawn

Could somebody explain how this works and how I increase numbers without having to mainly put 30 vehicles in myself everyday

It seems pmain is not spawning any vehicles after they have been destroyed. I started with 53 vehicles and now I have less than 30.
Thats a realy good question.
I have 80+ vehicles in my database, but the destroyed one didnt respawn.

There are some vehicles in the object_date with damage=1 and they didnt respawn.