The problem you face is because you are dis-aggregating the pie chart by putting 'Customer' in detail.
This calculation can be easily done using LOD expressions - This link provides all there is to learn LOD expressions Top 15 LOD Expressions | Tableau Software
The very first solution is 'Customer order frequency' and should provide the solution to your question.
I have also attached a solution workbook.
Hope this helps
Issue.twbx 15.9 KB