How can I set the prices of individual garments if I don't know the final production number?

Hello, for a calculation model I'm currently working on, I have fixed costs of approximately €10,000 per month. How can I distribute these fixed costs across an unforeseeable number of garments sold? What happens if I start with a certain number (e.g., 20 items sold monthly), but then sell more than that number? From the 21st part onward, my estimated fixed costs are "null and void," but are still included in the price.