I'm far from an expert on CPU architectures, but I suppose there are diminishing returns. Most of my functions use less than 16 values, and those that do may well iterate over more than 128.

Also you're probably aware that modern CPUs have many more physical registers, they just don't expose them all at the same time.

E.g. AMD Zen physically has 168 integer and 160 floating-point registers. Of which you can only use 16 at a time, but to my uninformed eye that looks like a reasonable trade-off.