First, you should not be running an action to import data from a List, it should be from a view where proper filtering can take place (this is not available when doing List to List imports).
On to your question...You have to be careful with Rank as it has a limit of 10 million cells (which you hit) because after 10 million cells, the Rank function can hinder performance. The reason you are getting this error is in the module you have the rank function, you probably have another list in the Applies To which is not the same (in volume) as in your source model. Thus, the overall cells in this calculation (in the target model) exceeds the threshold.
Cell Limit - The RANK function could be slow to evaluate at large data volumes, due to performance characteristics of sorting an arbitrary set that takesO(n * ln(n)). An artificial limit of 10M cells is imposed to prevent ranking of large datasets that would slow down the server: if this is exceeded, the model is rolled back. Note that when used against datasets of more than 1 million cells, any change in source values could result in calculations that take a few seconds.
So, how to get around this? In order to answer this, we would have to know more about what you are ranking. Does the Rank line item really need to be across multiple lists or can it be by only one list? If so, put the rank in a System (SYS) or Properties module with the Applies To being the list needed.
Export the list from Anaplan into Excel and you can do a count on that. As for the one in Dev, create a module with just that subset with a single line item named Count. If you have a top member on the list, you will be able to see the total.
Rob, thank you for your cue on importing from the saved view, apparently there was a module with saved view from where import into a list was happening, but somehow that saved view had got corrupted in the filters & pivoted columns