Confusion with data validation (list)

If you just want to calculate the number of days in a month based on the year in cell C8 and the month in cell C6, I suggest you try this formula:

=DAY(EOMONTH(DATE(C4,C2,1),0))

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Besides, if you want to select the corresponding data in the Data worksheet, I suggest you try this:

=COUNTIFS(Data!A5:A63,Test!C8,Data!C5:C63,Test!C6)

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Data worksheet:
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Update： Please check if the following is helpful to you.

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