SHEETS([reference])
=SHEET(Sheet1!A1)
The SHEET function returns the sheet number of the supplied cell reference. This returns the number 1 because Sheet1 is on the first sheet of the workbook.
=SHEET(Sheet2!A1)
Likewise, you can use the SHEET function to return the sheet number of another cell reference. This returns the number 2 because Sheet2 is on the second sheet of the workbook.
=SHEET(Sheet3!A1)
You can also use the SHEET function to return the sheet number of a third cell reference. This returns the number 3 because Sheet3 is on the third sheet of the workbook.
The SHEET function is used to return the sheet number of a reference sheet in a workbook. It includes all worksheets and other sheet types in a workbook.