Good question. I've just checked some blank load sheets (E175 & E190, so smallish aircraft) and can see that if you were loaded at the aft C of G for landing, on an E175 moving five passengers from the front to the back would put you outside the published loading envelope. Approximately 10 passengers would have to move on E190 to achieve the same effect. The effect on takeoff is less because the C of G moves aft as fuel is burnt. One hour's fuel is equal to about two passengers sitting in the front. And in case you are wondering, the fuller the aircraft, the more likely the C of G is to be in the centre if the envelope. This is due to the design of the aircraft. Your problems start when you fly just a handful of passengers with unusual freight in the holds. But given that we are aware of C of G problems when lightly loaded, we pay close attention to the initial C of G and "blocked" rows.