1 station requesting 100 unavailable stacks is less urgent than 10 stations each requesting 10 unavailable stacks.
I'm not sure about that, missing 100 stacks of Iron in the main smelter will grind everything else to a halt.
I'm also not sure how I could implement number of stations where n is missing without one additional virtual signal per item, or an entire new set of combinators.
For that you'd be much better off using LTNM https://mods.factorio.com/mod/LtnManager