The higher the number, the smaller the
needle. Smaller needles, such as the 24 and 16 are great if you're trying
to get your need through a small bead. The size I use most often are
the 20 and 22 which are perfect for 4-ply waxed linen thread. Don't
lick the thread, just flatten it between your fingernails and slide
it through the eye!
Size 16 is perfect for sewing ribbon through a stab-bound book.