It's hard to say what the actual breakdown is, since Apple doesn't release those figures. There are a number of sources claiming to have more or less reliable estimates, with some showing the base model is the biggest seller, but other reliable sources for the 15 and 16 series iPhones show that in the US, for their entire year-long release, the Pro Max might have sold better than any of the other models, though global sales are less certain, with some figures showing the Pro Max sells the most worldwide too, while other figures show the base model sells the most, with the Pro Max only a couple percentage points behind.
Note also that the three articles linked above are all from Nov 2024, when sales figures for the 16 series over its full year release period were especially unavailable yet since it had just been released only two months before. Often initial iPhone sales favor the lower-cost models. Later quarterly reports in 2025 show that the 16 Pro Max pulled ahead again of the base model, but not by as much as with the 15 series.
In any case, Apple sells enough Pro Max models to show that they should keep making more.