There should be a max combo right next to the current combo. I don't like to have the scoreboard on to see my max combo, I sometimes accidentally press tab when I do that I try to press tab again but it makes me miss or combo break.
the only way to know the max theoretical combo for sure on maps is the editor while testing the map (because auto can have issues...), i think it would be appropriate if this was displayed on the map along with the SR, difficulty settings, etc. this max combo thing also seems like it would be wiser if it were a skinnable element instead where you have options on how it displays in game.