I was thinking about a mod which implents a new resource: Manpower.
I want to gain automatically (maybe 30-45 seconds) 1 manpower. You need it for every soldier.
But I'm a newbie at modding so I don't really know more than the basics.
Maybe anyone can teach me how to do it

A manpower resource wouldn't be awfully difficult. Start a timer when you begin a scene, then add 1 to manpower every x seconds. The hard part would be including a check for the number of bodies in incoming craft and autoreturning them if they exceed manpower.

Just make a table when that checks for every actor on the scene that's on your team, if the amount of actors exceeds the manpower then delete the most recent actor added to the scene.

Table's are a weird and tricky thing to handle with and takes practice, but that's how I would do it.