I fly the B717, and we require at least 2 per row at the overwing exits, if the total pax numbers exceed 31 (if less than 31 we don't need the minimum 8). Nothing is to be on the floor or under the seat in front and we've even recently added a rule that they must keep their shoes on for take off and landing too (as for some reason many of my dirty smelly passengers love to take off their shoes the moment they sit down...even using the toilet in bare feet *shudder*)