When I know how many of each item I am sewing, I work on the materials lists. I am currently about half way through figuring out what each piece is going to take in reference to fabric, time and other materials.
Sometimes by making multiples from the same fabrics, I will use less materials per piece than if I only did one. It will be difficult to figure that reduction into the overall needs, but I will probably just have leftovers.
“My scrap bag is almost empty and I need more scraps to put into it.” Said no quilter, ever!
I will also have to consider whether the same supplies can be used in different Christmas sewing projects. I am sewing pillowcases and earbud cases. I will be able to use some of the same fabrics for both groups of gifts.
All in all, this process is a guessing game at best. The key is to buy too much, rather than too little.