There are different amounts of pre-cuts depending on the brand, and it depends whether you are seaming them on the short edge or the long edge.
RK rainbow jelly roll has 41 strips that are 44" long. If you pieced only on the long side, your quilt would be 43x164" on the short side would be 86x82". Of course you could always choose to add 2 more strips or crop the edges if you want it perfectly square (actually the strips are not always identical widths anyways so there would likely be cropping regardless).