Array formulae/macro? Counting datasets -

Try in D6

=IF(AND($B6<>"",$B7=""),COUNT($B$6:$B6)-SUM($D$5:$D5),"")

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[EDIT]

This seems, to me at least, to better answer your problem.
Using RANK() as a condition could be a bit difficult or perhaps just misleading.
What would happen if the data-set has more than one value ranked first?

Try this

Sub StuffToDo()

    Dim Rws As Long, Rng As Range, c As Range
    Dim Rnk As Range, y As Range, Srk As Range

    Rws = Cells(Rows.Count, "B").End(xlUp).Row

    Set Rng = Range(Cells(1, 2), Cells(Rws, 2))

    For Each c In Rng.Cells

        On Error Resume Next

        If c = "Data" Or c = "MS" Then

            Set y = Range(c.Offset(1, 0), c.End(xlDown))

            Set Rnk = Range(c.Offset(1, 1), c.Offset(1, 1).End(xlDown))

            For Each Srk In Rnk.Cells

                If Srk.Offset(0, -1) <> "" Then

                    Srk = WorksheetFunction.Rank(Srk.Offset(0, -1), y)

                End If

            Next Srk

        End If

    Next c

End Sub

Here's a formula approach, I perhaps missed that you also needed the ranking of each set.

In Column A (Helper Column)
In A6

=IF(B6="","",IF(ISTEXT(B5),ROW(),A5))

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In C6

=IF(A6="","",RANK(INDIRECT("$B"&ROW()),INDIRECT("$B$"&A6&":$B$"&COUNTIF(A:A,A6)+A6-1),0))

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In E6

=IF(C6=1,COUNTIF(A:A,A6),"")

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I have left my original suggestion in Column D for reference only, it isn't needed with the other formulae.

This assumes that each of your data sets has a text header in Column B

Hope this helps