Well, you've pretty much summed it up!
Depending on your bench capacity, you should have at least 2 backup RB's and 2 backup WR's. You'll need a backup QB for sure. If you have another spot, there's nothing wrong with having a handcuff RB or possibly a TE if they are worth it. That's about it.
Yes, RB's are a vital commodity...but you still have to think about your other starters. If no one wants to trade with you, most of those RB's will go for nothing except better matchup starters for you. That will end up hurting your other starters.