Counting unique values across multiple tabs

Samphire 

Here is a custom VBA function you can use:

Function CountUnique() As Long
    Dim wsh As Worksheet
    Dim r As Long
    Dim m As Long
    Dim c As New Collection
    Application.Volatile
    On Error Resume Next
    For Each wsh In Worksheets
        Select Case wsh.Name
            Case "Summary"
                ' Skip this sheet
            Case Else
                m = wsh.Range("A" & wsh.Rows.Count).End(xlUp).Row
                For r = 2 To m
                    If wsh.Range("F" & r).Value <> "" Then
                        c.Add Item:=1, Key:=CStr(wsh.Range("A" & r).Value)
                    End If
                Next r
        End Select
    Next wsh
    CountUnique = c.Count
End Function

Use like this in a cell:

=CountUnique()

Save the workbook as a macro-enabled workbook and make sure that you allow macros when you open the workbook.