Product quantity fro Limited Edition print

I'm looking for someone who might have the skills to code this, because im not interested in installing any apps. If it's possible.
I'm selling Limited Edition prints, in 2 collections - 1 Colour & 1 Gold.
The Colour collection i'm selling 30 of each, and the gold is set to 20 each, and 3 different sizes.
So I have to set the quantity for ex. 10 small size, 10 medium size and 10 large, for the Colour.

My wish is to have 30 in total, no matter what size you buy, if that makes sense.
So I dont want to 'limit' myself to 10,10,10, but rather put 30 for each product in the Limited Edition, and no matter what size the costumer buys, it just subract from the total of 30, and 20 for the gold.

And would it be easy, if there is a solution, to set it up in the future whenever I get new Limited Edition products?

Hope to hear from you, if this is a simple task.

Thank you very much in advance!