These lines of code:

and

(also referenced, here:

limit multi-channel sampling to a sequence of max length 5. The STM32H7 actually supports 16 channels (accordingly #defines up to ADC_REGULAR_RANK_16 exist), and arguably the feature is most useful when more than a handful of pins are involved.

Now, I am not quite sure what is the long term plan for cross-hardware support in this library. Depending on that, the proper fix will either be a plain adjustment of the values, or a small mess of #ifdefs.