Got it work.
Here's the code:
Sub MaxColNum()Dim rng As Range
'convert column Letter to Number and find the Largest column
Dim LastColumn As Long
Dim LastRow As Long
With ActiveSheet.UsedRange
LastColumn = .Columns(.Columns.Count).Column
LastRow = .Rows(.Rows.Count).row
End With
'Clear the 'cell Named MaxLC. in the end of the the BIGGEST LETTER will be placed on this cell
Range("MaxLC") = ""
'set the Range to Loop
Set rng = Range(Cells(2, 3), Cells(LastRow, LastColumn))
BiggestLetter = 0
For Each ColN In rng
If ColN <> "" Then
'Convert Column Letter into column Number
'and load the number into ColN variable
ColN = Range(ColN & 1).Column
If ColN > BiggestLetter Then BiggestLetter = ColN
End If
Next ColN
'Placed the BiggestLetter found into cell Named: MaxLC
Range("MaxLC") = BiggestLetter
End Sub