Initially it used to be lesser than 10 Million, they have increased it to 10 Million in Q2 2018 but this still is a limitation - I guess to avoid performance issues. However I would like to ask if you you are trying to rank in multidimensionality or Is it just 2 dimensional where you dimension is too big for Rank function to perform. Can you please post your requirement and the way you have coded it.
In my opinion you should at first transform your module with 3 dimensions to module with 2 dimensions only, and only then use Rank function.
The easiest way is to make view in current module that filter only the calendar date you need, and then import it to new module with action. It would make from 300x to 1000x values less. It would be less than 1mln cells for rank function.