Click "File Manager" and navigate to mpmissions/<Mission_Name_Here>/db/events.xml
Click the pencil icon to edit the file.
You need to edit the nominal, the min and the max values for each vehicle.
After reaching <min> items and <restock> seconds, the spawner will drop <nominal> items on the map.
After reaching <min> items and <restock> seconds, the spawner will drop <nominal> items on the map.
<nominal> needs to be lower than <max>.
Make sure to save the file after making the edits!