In your example, are you looking for the top 50 "Customer ID"'s based on Sales or top 50 "Customer Names" ?
Right now you are sorting Customers by their sales amounts and because you have Customer ID in the report, the way you have it setup it's going to show you the top 50 Customer ID records for customers based on their sales.
If you're trying to show the top 50 customer ID's, swap Customer ID to be the leftmost on the rows shelf and apply the Sales sort on that field, not on the customer name.
If you're trying to show the top 50 Customer Names, edit your table calculation for Rank and change it to 'Compute Using' > Specific Dimensions > Customer Name and Customer ID selected, change 'At the level' to Customer Name.
If you're trying to show the top 50 customer ID's WITHIN the customer name, edit the table calculation for Rank and change it to compute using > specific dimensions > Customer Name and Customer ID selected, change at the level to Customer ID and restarting every to Customer Name.
You could also use a different approach by using the TOP n filter:
If you're looking for the top 50 Customer Names, remove the RANK filter and drop Customer Name onto the filter shelf, then click the top tab and set it up like: