My guess is it is short for numbers. In English No is short for number so nos would be numbers. It is used a lot in the building trade where measurements can be mixed up with the quantities as they are both numbers so you say 3 number 10 by 2 plyboard and it becomes clear that the 3 is the quantity. It’s unusual in the UK or Ireland to see it pluralised ie nos instead of just no for all numbers. It has made it’s way out in to other fields where ambiguity might be an issue. Having said that I find “each” more common when the number of single units as the typical use case here. I’d just set your default to each if that’s what makes more sense.
Just waiting for someone else to come along say well actually it means this “?” 