not all jelly rolls come with 40 strips, they are usually the strips from one collection, so if there are 26 fabrics in a collection you get one 2 1/2" strip of each, so 26 fabrics. if you are going to make your own strips you can use as many or as few fabrics as you want to.
after cutting off salvages your strips (if you are lucky) will measure 40" long...it would take 30 strips to get 60" wide quilt by 40", if you want longer you would need more strips of course to add to your 40" lengths. 45 strips (cutting 15 in half to add 20 more inches to the 40 inch length will give you 60"x60".