I think each string of beads counts as its own door. Hear me out. If you had a double door, you’d count it as two doors, right?

If you were living in a cave and you used an object to block the entrance, that’d be a door, right?

Ergo many objects that together serve as a door just by being in the way each count as separate doors.

But if you had an accordion door you’d only count that as one, so that’s why it’s each string of beads and not each bead.