Last Breath has 4 phases.

 

A comparison of Last Breath, classic Sans, E10, and E20.

Classic Sans has 2 phases. A lot of people seem to agree on that. That despite the two phases sharing the same design, same music, same theme, the classic Sans fight is split into two distinct halves. Phase 1, and phase 2. Or, phase 1a, and phase 1b (though, without other phases, it's called phase 1 and phase 2).

Here's the thing. Classic Last Breath has the exact same split. The phase without orange attacks (phase 1a), and the phase with orange attacks (phase 1b). Why, then, are these not usually considered separate phases in the context of Last Breath? Especially given they're both about the same length as phase 2, and as the phases in other Last Breath AUs. I personally think they should be considered separate, and that Last Breath should be considered to have 4 phases. Just because phases in fights in the original game are simply distinct sections of the fight - they don't need to have music, designs, etc. The only character for which that is true is Asriel. (though even then, one could argue that his second form includes 3 phases).

Thus, Last Breath has 4 phases: phase 1a, phase 1b, phase 2, and phase 3.