For ideal gases, the speed of sound is independent of pressure. So all that matters is the temperature and the fact that it’s largely CO2 instead of N2/O2. The fact that the pressure is 0.5% of Earth basically drops out of the formula.

Rotors can go supersonic, but that poses all kinds of problems (shockwaves, etc.), and isn’t too efficient. Ingenuity keeps its rotor tip speed under mach 0.7, which probably keeps the gas flow under mach 1 across all surfaces of the rotor.

If tip speed is the limiting factor, a large rotor gets easier–centripetal acceleration is \frac{v^2}{r}, so if v is constant, then the 1 \over r factor reduces the accelerations. But a large rotor also needs better stiffness to not sag, and is going to be harder to fold up for storage. I think this latter point might be the most significant factor; a multirotor can fold up in a way that doesn’t take much space.