There are between 20-30 RBs picked most drafts. Here are the last 5 years:
http://pfref.com/tiny/mxWKk
Two things standout to me:
1. RB is in the middle of a resurgence. Teams have started picking them early and often again. Backs like Bell, Elliot, Gurley, David Johnson have demostrated that you can have a 3 down RB dominate in a modern offense. Something that a few years ago many believed was no longer possible.
2. RB value is ALL over the map. David Johnson went after Duke Johnson. Marlon Mack was billed as a great prospect and so far he isn't as good as Elijah McGuire who went later.
So if the Steelers really did want to draft to replace Bell, talking a Round 1-2 pick (RBs are rising in value across the league) and maybe needing to take more than one guy - sorta like the Packers last year. All I'm saying is that now your draft looks locked in to needing to pick a RB and ILB in some combination in the first two rounds. Not a fan of having roster needs force your hand on draft day. But there are lots of other considerations as well...