Easiest way is to look for a jelly roll quilt pattern that comes in different sizes where the creator has already done the math for you...How many you need is dependent of the specific pattern as the more seams there are the more jelly rolls it is going to take. You can also sometimes add to the fabric from your own stash if you have something that will fit in nicely. I have done that when I have been a little short on Jelly rolls. If you have a pattern for a twin sized quilt and want to make a king sized quilt, you can count the number of blocks in the twin sized one and then figure out how many blocks you will need for the king sized ones and add in the number of jelly rolls you will need.