Each bowl of food and milk will feed ONE breeding cat for FOUR weeks ---- or Four cats for One week. So 20 cats will need 5 bowls of food and 5 bowls of milk each week. The milk is not actually required. That simply decreases the time it takes for a cat to go from 0% Love to 100% Love. You can achieve the same thing by cuddling each cat for 30 min per day (you can hold more than one cat at a time). With milk/cuddles it takes 7 days to get to 100% -- without, it will take 10 days.
The manual for this:
https://kittycats.ws/manual/category.php?id=21
Buy the 20 packs and those will last for 4 weeks for 20 cats.
Side note -- Yes, it gets expensive, but the hope is to recover at least a good portion of your costs by selling the kittens.