To the OP:
The Model 12 doesn’t have a dedicated control room bus that splits the headphones out like the M16/24. So in most cases the headphone volume is driven by the master fader.
So per Mark above in many cases if you plug your monitors into Main out, you just have to deal with it by adjusting headphone volume and or volume on you external monitors. Also caution, note if you depress the “Post Rec” button to say add some whole mix EQ while recording, the master fader also impacts the volume recorded to tracks 11/12, your master tracks. So yeah, all tied together.
BUT, you do have the option of plugging monitors into Sub outs. This gives you a slightly quieter signal, and a separate master fader control over your mix volume... just need to assign the tracks to sub and master via the buttons at the base of the fader.
And of course you can create an aux mix (for say stage monitors or cue mixes) and easily switch the phones between main and aux. you could just set Aux 1 knobs to 0db and hear a mono mix of your tracks pre fader.
And ... hey, there is a very cool feature that can give you some of what you are perhaps looking for. I recommend you spend some time exploring the difference between setting the Mixer (menu) solo choice between PFL and In Place (AFL).
*** In PFL solo mode, the headphone mix is DECOUPLED from the master mix when you solo a track. So you will hear the solo track through the phones post EQ but prefacer without changing the master out. Nice, eh?
*** In Place mode, the headphone and master bus are coupled, and both get only the. Solo track, post fader. The lights display the other tracks as muted.
I really like this feature - i regularly switch the solo mode depending on what I am doing. once you crack it, the M16’s PFL-only action feels limiting.