I received a mix the other day, together with a description of how the mix had been created. One sentence stood out… “Master processing was kept to a minimum.”
Well this set a train of thought in motion. How much master processing is the minimum required?
One simple answer is none. If you can balance all of the tracks perfectly and achieve a mix that is professional-sounding in terms of balance of frequencies and dynamic range then you really can get away with no mastering at all. Indeed it is a worthy aim to try to craft your mix in this way.
What happens more often however is that although all the tracks blend together in terms of relative balance of frequencies, the overall balance of frequencies isn't compatible with the needs of the market (in other words, it doesn't sound like commercially-successful releases).
This can easily be corrected with a little EQ in the master track. And even if you get the EQ wrong, it can be corrected by an experienced mastering engineer with no audible loss of quality.
Putting any kind of dynamics processing into the master track does change the audio irrevocably. If what you do is good, then no problem. But not even the best mastering engineer in the world can correct any errors without access to the original multitrack recording.
To draw to a swift conclusion, then the minimum mastering processing is none, but it is likely that you will need some EQ. So for practical purposes this is the minimum master processing required for a successful mix.