I didn't say to raise the tracks up to +12 or +20, I said to LOWER the levels of all the tracks to around -12 [MINUS 12] or -20 [MINUS 20].. I'm sure you've noticed that the volume numbers for the tracks are either positive or negative -- a level setting of -6dB is 12dB lower than a level setting of +6dB.
The point is that you need to lower the level of all the tracks, so you can raise the level of the piano track to the level you need. For example, let's say you have a song with three tracks -- a Cello, whose volume is set to +2, another instrument whose volume is set to +1, and the piano, whose volume you've pushed up to the maximum of +6 but still need more level. What you must do is lower the levels of all the tracks by the same amount, So, for example..
- Lower the level of the Cello from +2 down to -10 [12 dB lower]
- Lower the level of the other instrument from +1 down to -11 [12dB lower]
- Lower the level of the piano from +6 down to -6 [12dB lower]
- Raise the listening level of your interface/speakers to compensate for the drop in overall playback volume
Now your mix should sound exactly the same, but since the piano is now set to -6dB you'll be able to increase the level of the piano, since it's no longer maxed out.
A complication is that if you have automation on then to make those adjustments you'd have to drag the automation line[s] rather than move the volume slider[s]. Are you changing the volume of some tracks at different parts of the song? If not [if the automation line is just a single straight line], I would recommend that you turn off automation on all tracks, set the levels with the volume sliders for each track, and then make the adjustments as I described in the example above.