There isn’t a single hard maximum ISO in the usual camera sense, because ISO is largely just gain applied to the captured signal. In principle, you can keep amplifying the recorded signal as much as you like.

The real limit is usefulness, not a fixed ISO number. As light levels get extremely low, image quality is dominated by photon shot noise: photons arrive randomly, and if a pixel only receives a few photons, the uncertainty is a large fraction of the signal. No amount of amplification can recover detail that wasn’t measured in the first place.

So the physics-based limit is tied to detecting very small numbers of photons, potentially down to single-photon events in an idealized case. Pixel area matters because larger pixels collect more photons in the same exposure, improving signal-to-noise ratio.

In short: there’s no strict upper ISO ceiling set by gain alone, but there is a practical limit set by photon statistics and sensor noise. Beyond that point, raising ISO only makes a very noisy, very weak signal brighter—it does not create more information.