How to show total count from sheets when using multiple drop down lists in Excel?

I have a workbook with multiple sheets. On each sheet you can select a user from a drop-down list and then select an item from a different drop-down list. There can be multiple instances of user and item on any sheet.

I need to have a summary sheet where you can select the user from a drop-down list and excel will display the sum total for each item the sheets.

I am able to collect single results using the following formula:

=COUNTIFS('15'!$C:$C, E4, '15'!$D:$D, E5)

My sheets are titled 1 through to 31.