12” blocks you need 100 blocks ( 10 across, 10 down) to make a king size quilt.
if starting with a 2 1/2” square in the middle of each block you can get 16 starting squares from on 2 1/2” x 40” strip.
There is a lot of math to do to figure jelly roll needs. I would use a piece of graph paper ( or just sketch out your block) with the measurements for each strip ( remember seam allowance) then see how many of each strip length you can get out of each jelly roll strip, then you can figure out how many strips you will need for 100 blocks.
I just did the block- math. Using 2 1/2” Center. - first add 2 1/2” x 2 ; 4 1/2” x2, 6 1/2” x 2, 8 1/2” x2, 10 1/2” x w, 12 1/2” x 2 completes one block
you would need 288 2 1/2” strips. So, depends on how many fabrics are in the jelly roll bundles.
if there are 40 strips per roll you would need 7.2 ( you will have pieces left over to use so probably only need 7)
if there are more than 40 strips divide 288 by the number in the roll and that tells you how many rolls you need