Bundles are wefts of hair. They cover the back and sides. They cannot cover a parting, which is what the closure or frontal is for.
A closure is a small lace piece, usually 4x4 or 5x5 inches, that covers the crown and gives you one parting. A frontal is wider, 13x4 or 13x6, and runs ear to ear, which gives you a full hairline and the freedom to part anywhere or pull it back.
Three bundles cover most full sew-ins to about 18 inches. Past 20 you want four, since longer hair is cut from the same weight and thins through the length. With a closure, two bundles are often enough at shorter lengths, and a frontal covers more of the head itself so the count comes down again.