"Item Data (Key: Item Shortname, Value: Stacksize)": {
"hat.wolf": 2500,
"fogmachine": 2500,
"strobelight": 2500,
"kayak": 2500,
"minihelicopter.repair": 2500,
"scraptransportheli.repair": 2500,
"ammo.grenadelauncher.buckshot": 50000,
"ammo.grenadelauncher.he": 50000,
"ammo.grenadelauncher.smoke": 50000,
"arrow.hv": 50000,
"arrow.wooden": 50000,
"arrow.bone": 50000,
"arrow.fire": 50000,
"ammo.handmade.shell": 50000,
"ammo.nailgun.nails": 50000,
"ammo.pistol": 50000,
"ammo.pistol.fire": 50000,
"ammo.pistol.hv": 50000,

i want only x5 but not 50k ammo hv

Just change the Stacksize to a something you want it to stack to - also consider unloading StackSizeController, and then unloading MagicLoot, deleting the data/MagicLoot.json, then loading MagicLoot again so it will then load the actual default rust values instead of the modified StackSizeController values. These will then be saved so you can load StackSizeController back without it overwriting these MagicLoot values