[dredmorbius]

You've just made me realise that several of the more popular freewheel / sprocket combinations are in fact prime: 11, 13, 17, 23.

Usually contrasted against the decidedly non-prime 52 (2 * 2 * 13) and 42 (2 * 3 * 7) chainrings. 38 if you're old-school triple (2 * 2 * 7).

My chain lengths would vary and I never counted them specifically, though I'd typically remove a few links for fit.