Obtain COUNTA value in row of data, and place in Textbox

I am trying to obtain No. of used cells between columns 'L - AP" Column #(12-42) in the below code, but i only ever get a value of 1??
With current data, i should get a 6.

Private Sub ListBox1_Click()
Dim lastcell
Dim myrow
Sheets("SB_DATA").Visible = True
lastcell = Worksheets("SB_DATA").Cells(Rows.Count, "B").End(xlUp).Row
With ActiveWorkbook.Worksheets("SB_DATA")
.Select
    For myrow = 1 To lastcell
    If .Cells(myrow, 2) <> "" And Me.ListBox1.Value = .Cells(myrow, 2).Value Then ' Listbox has a value that is found in Column B(2) to set the row of data required to work with
          Me.TextBox2.Value = .Cells(myrow, 165).Value
          Me.TextBox4.Value = Application.WorksheetFunction.CountA(.Cells(myrow, 12), .Cells(myrow, 42)) ' <---- Trying to obtain No. of used cells between columns 'L - AP"
             End If
    Next
End With
End Sub

Thanks bakerman2, perfect.
I can work with that as is.