The Size Is Written Into Law
Migration Code article 36.1.3, and it asks for two.
Most countries publish photo rules as guidance on a portal. Azerbaijan publishes them as legislation: the Ministry of Foreign Affairs reproduces article 36 of the Migration Code, and clause 36.1.3 reads "Two color photographs (3x4cm) matching with biometric identification requirements".
Two things follow. The 3x4cm frame is the same one the ASAN e-visa uses, so a single crop covers both routes. But if you apply at an embassy or consulate rather than online, you need two identical prints of it, not one.
The phrase "matching with biometric identification requirements" is doing the work that a list of millimetres does elsewhere — it points at the ICAO geometry rather than restating it, so face square to the camera, neutral expression, eyes open and unobscured.