It depends...
I heard that the ASA RJ flying from ATL to FAY only needed 7 passengers to break even. However, on a longer, or shorter leg, these numbers can be different. Also, depending on pay rates, fuel costs, etc. The formula is NOT set in stone...it changes. What you need to do is call Bombardier, and see what their DOC (direct operating cost) is per hour, then see how many seats you need to fill ( and at what price per seat). There are MANY variables. Typically with airlines, fuel and crew costs come before others, however, at some of these regionals, crew costs might be lower on the list since they pay so little.