There are different draft styles, (Redraft, Keeper, or Dynasty), scoring methods, (Point Per Reception, Half a Point Per Reception, Standard, TE premium, and countless more), and roster differences (1 Quarterback, 2 Quarterbacks, Kicker, No Kicker, Defensive player, Super Flex, and countless more).
The standard roster in ESPN leagues (and most leagues in general) is one Quarterback (QB), two Running Backs (RB), two Wide Receivers (WR), one Tight End (TE), one flex, one defense/special team (D/ST), one kicker, and seven bench spots.
Example in a standard roster 12 person league you have 24 starting running backs with the potential to have 36 running backs each week, if everyone has a running back in the flex.
So in a 12 person league everyone can realistically draft a starting running back and put him in your starting lineup, while having starting or running back by committee player in your flex spot or on your bench.