Eh? Huh? Runs out or bits? Loudspeakers, and audio amplifiers are analog devices. SPL is an analog measurment. You can't "use up all of the bits" by playing your system too loud. In fact, "bits" has nothing to do with it.
Rutgar, we're talking about the level at which the sound is recorded on the digital recording (e.g., DVD). I believe the number of bits determines the frequency extension possible and the dynamic range available (dynamic range is what we're talking about).
If you're audio system is calibrated to reference level, and you play back something recorded at the highest (peak) level, that is the limit we're talking about.
You can always play it louder by turning up the volume on your amplifier, but you can't make it any louder by recording the signal at a higher level on the disc. It's already at its maximum.