If one bead is 2 inches, think of it like adding 2 each time one bead is added until you reach 8,
2 + 2 + 2 + 2 = 8, yes?
As you can see, there are 4 2's, so there are 4 beads that can fit onto the 8 inch string.
A faster way to do this is to do:
8 divided by 2 = 4
HOPE THIS HELPED :D