If you have a pre-printed pattern, it should tell you how many to buy. If you are making it up, just decide how big the blocks are going to be, then figure how long each strip needs to be for that block. Multiply that by the number of blocks you want and you will get the approximate amount of fabric you will need. Now, most jelly rolls have 40 strips of approx. 40"-41" each. So you will have 1600 inches by 2.5" (before you include the seams). Now the rail quilt doesn't lend itself to using a jelly roll as usually you use 3 or 4 strips of the same fabrics and just keep turning them. You wouldn't want 40 different fabrics. After reading this, I hope it is not too confusing. I just finished a queen size quilt for a man I work with and I think I used close to 4 jelly rolls not counting the borders.