Customer ID Count Report

It's common for analysts to count the number of customers via a customer ID captured with a prop or an eVar instead of using unique visitors. It'd be great if was a report that was a sum of the unique counts within a particular variable. e.g. we use eVar 12 to capture our unique customer ID. When we calculate we penetration for reporting purposes we manually count the number of rows in eVar over a given period. Ideally, there would be a seperate variable that would sum the row count from our eVar 12 customer ID variable. That would also enable customer ID counts for various breakdowns and report automation.