There are around 15 RBs in the top tier or two that contribute. Most people try to draft them in the first two rounds. After that, RB production drops off a cliff. So if you don't have one or two of those top 15, you're toast since your two RB slots aren't producing enough. This is why people say RBs win leagues because you need some of the decent ones to be competitive.

This tends to be more true in traditional leagues than in PPR leagues. If you're in PPR you can get away with more WRs and third-down type RBs like Hunt, but still need at least one top RB.