It is very kind of you to do this. Many people have come to different projects, some very simple and some more complex but I've seen more than one project made from shirts -- and a lot of those were plaids, some were more work style and some more hunting style.
Fortunately shirt backs are often a nice piece of yardage that is usually in good shape and stain free! Figure you will get at least a fat quarter per shirt, probably two, I'm doubting though that you would get a yard per shirt. So I'd want to start with at least 12 and figure 20 would be a reasonable cut off point for woven/non t-shirts.
Search on "memory quilts from shirts" for ideas. You'll get a lot of the t-shirt links but not all.
Sadly, my father in law passed before I got this quilted down, but here's a top I made of plaids for him. It is a traditional "Delectable Mountains" block arranged around squares of the focus plaid. I would think you could get one set of something like this from a long sleeved shirt in good shape.
https://www.quiltingboard.com/member...14-621635.html