I freeze them all on cookie sheets separately, then put in bags then in plastic containers. This freezes them faster (better flavour and texture) and allows me to grab the exact number I need.

I bake the backs and skin and fat in a large pan. I add bits of onion. I brown it all a bit. Then I make a stock, about 8 to 12 quarts. I get it hot as soon as possible (start on med-high to high) with cold water. Turn it down when it just starts to boil. After six hours I add carrots, celery and onions and a couple bay leaves. I usually save vegetable trimmings when I am cooking. At seven and a half hours (after start of boiling) I add crushed pepper corns and a little salt.

After eight hours I turn off the heat and put it on a cold granite cutting board. Cool for 30 min. I strain the liquid and fat into another pot. Then I put it in the sink with cold water and ice packs. Cool as quickly as possible. Then I put it all in the fridge. The next day the fat has solidified on top. I remove it and through it out. I divide the stock into containers and freeze it. I use yogurt containers, 750ml, 125ml and ice cube trays. This covers all the different amounts I use in different recipes.

Note that you need to get the temperature of the stock up quickly and when you are finished, down quickly to reduce chances of food poisoning.

When I was working in a restaurant another cook took the pot off the heat just as it was warming up. He forgot to put it back on. By the next morning the pot was ‘boiling’ at room temperature, and it was the foulest, most gruesome, disgusting concoction I had ever had the misfortune of encountering.
I made the other cook clean up the problem.