Nasário... veja se é isso:

Function NumUniqueValues(Rng As Range) As Long
Dim mycell As Range, UniqueVals As New Collection
Application.Volatile
On Error Resume Next
For Each mycell In Rng
If Rows(mycell).EntireRow.Hidden = False Then
UniqueVals.Add mycell.Value, CStr(mycell.Value)
End If
Next mycell
On Error GoTo 0
NumUniqueValues = UniqueVals.Count
End Function

Edson,

Funcionou com a alteração abaixo,

Function NumUniqueValues(Rng As Range) As Long

Dim mycell As Range, UniqueVals As New Collection

Application.Volatile

On Error Resume Next

For Each mycell In Rng

If Not mycell.EntireRow.Hidden Then
UniqueVals.Add mycell.Value, CStr(mycell.Value)

End If

Next mycell

On Error GoTo 0

NumUniqueValues = UniqueVals.Count

End Function

Valeu demais!