You will find great info from Bonnie K Hunter on this very topic. Her website is
www.quiltville.com. She has a book, Scraps and Shirttails, from which I just finished making a quilt from my FIL's shirts. He died a couple of years ago and I asked my MIL for his shirts. She gave me 13 of them. I was able to make a 63" X 81" throw (did have to add a couple of yards for sashing) and had enough shirt material left over for a second quilt which I'm going to make for my DH for Father's Day. I will ad some other yardage of airplane print to his, but only a little.
Bonnie says you can get about 1 1/2 yards per shirt. She has a video on her website about taking apart a shirt. Acraps and Shirttails Too.
If you add any fabric, be sure to prewash it because the shirt fabrics will all have been washed and you don't want some of you fabrics to shrink and others not!!