|COUNTA is not for counting occurrences of a given value.|
Per the Excel help files:
COUNTA Counts the number of cells that are not empty and the values within the list of arguments. Use COUNTA to count the number of cells that contain data in a range or array.
To count the number of occurrences of a given value in a range you would use COUNTIF.
Unfortunately, COUNTIF does not work across multiple sheets.
One method that should work for you is to use a COUNTIF in a cell in each sheet and then SUM those cells in your summary sheet.
My method (there are others):
I'll assume that the Summary sheet is the last sheet (Sheet16)
Group the sheets by Clicking the tab for the left most sheet (Sheet1) and then Shift-Click the tab for Sheet15.
In Sheet1, in a cell you know to be empty in every sheet (e.g. A1) enter:
or any formula that will return a 1 if G3 contains an "x".
This will enter that same formula in A1 on every sheet.
In your summary sheet enter =SUM('Sheet1:Sheet15'!A1)
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