Abstract: How many QCD axions can there be? Can the standard ma–fa relation be modified without extra sources of PQ breaking? Multiple axion-like fields arise naturally in string theory, extra-dimensional models and other SM extensions, with implications for the strong CP problem, dark matter and experiments. I will present a novel sum rule linking their individual signals as a rigorous consequence of the PQ symmetry, the significant modification of axion dark matter production through level-crossing effects, described analytically via the Landau–Zener formalism; and whether a next-generation helioscope such as IAXO could tell one axion from two, exploiting the spectral signatures of axion-flavour oscillations over the Earth–Sun baseline.I will then briefly outline two further lines of my work. First, the use of Hilbert series to systematically construct effective field theories and expose their (accidental) symmetries, with applications to flavour. Second, unitarity and long-range interactions: already at tree level, partial-wave amplitudes become ill-defined in the presence of a massless photon (a t-channel singularity that obstructs unitarity bounds), and we recently showed how to resolve this long-standing issue, recovering finite, unitary partial waves. This has direct application to SMEFT amplitudes involving the Standard Model photon.Based on arXiv:2305.15465, 2507.06287, 2606.20826 (multiple axions); 2312.13349, 2412.05359 (Hilbert series); and 2510.08784, 2606.19432 (unitarity).