Let's say the maximum output is +28db... now if you run a mic signal in and get it to to sit around 0db, you then have 28db of headroom to work with -- meaning your signal source has 28 db of "space" in which to peak, so if you're recording drums, if that cymbal splash causes a 30db peak, you ate into your headroom by 2 db....
Headroom is not so much an absolute reference as a relative one dependent on the dynamic range of the device and the signal level being sent thru it.
So I can't answer your question with numbers without knowing 1) the overall dynamic range of the unit, and 2) the signal level you're applying.
Bruce