Depends on the torque specs of your lug nuts and the impact wrench you're using. If you don't have an air compressor and are only planning on using it for an impact wrench, I suggest getting an electric cordless impact wrench instead, as it will use less space and require no maintenance compared to a compressor and air impact.
If you're planning on using the compressor for other stuff, then I suggest at least 4.5cfm at 90psi, more if you want to do sand blasting or grinding continuously.
edit; if you're wanting to use a cheap but powerful air impact like the HF Earthquake 1/2" gun, you want closer to 6CFM output at 90psi. Cheapest compressor you can get that will push that, new, is at least $350.