For example:

Sub HighlightLast()
    Dim r As Long
    Dim m As Long
    Application.ScreenUpdating = False
    m = Cells.Find(What:="*", SearchOrder:=xlByRows, SearchDirection:=xlPrevious).Row
    For r = 21 To m Step 20
        Range("A" & r).Resize(1, 3).Interior.Color = vbYellow
    Next r
    ' Last row
    If m Mod 20 <> 1 Then
        Range("A" & m).Resize(1, 3).Interior.Color = vbYellow
    End If
    Application.ScreenUpdating = True
End Sub