can't find a property that does this e.g 
minloot: "1"
maxloot: "5"
does something like this exist, if not how do I set this ?

Thanks

hi, it's in the profiles, oxide/data/RaidableBases/Profiles/

Amount Of Items To Spawn

I increased that but then I find my boxes only have 4 items in them, I would like boxes to have say 6-8 items each ?

ya, that is how you increase the loot the in the base

if you don't have enough items in your loot tables, or the priority is too low then you won't have enough items spawning

you need to add more items, or enable Allow Duplicate Items