Quite a bit. The lift produced by an airfoil is proportional to air density. You can compensate by turning the rotors faster, increasing the size of the rotors,

Quite a bit. The lift produced by an airfoil is proportional to air density. You can compensate by turning the rotors faster, increasing the size of the rotors, and/or increasing the angle of attack. None of these options are free and all of them have limits.