Ok, to what number, and why?
It hard question. I newbie in game, and i think i cant give impersonal opinion
I think good resolve be use random and depending on the location (and, obviously, different things in different amounts - depending on the usefulness)
for example (nmb to sell)
assault rifle 4-6
assault rifle magazine 20-40
sniper rifle 3-4
sniper rifle magazine 16-20
riot shotgun 2-3
saboted slugs 8-12
flechette shells 8-10
medikit 3-4
IR goggles 2-3
frag grenade 3-5
flashbang 2-3
smoke grenade 1-2
combat armour 3-4
and price twice more then manufacture
also it can differ by difficulty (on easy - more items for sale, on hard - less)
manufacture to beat in terms of what the world was not enough weapons, and that would take time to get it right (ie, not manufacture as literally, it mean - search for weapons required in world)
it also allows you to add weapons to the store for sale (by twice price)
such a request was made for 3 rifles, found 5 - 2 rifles fall into the shop