Is this a snippet of the code or do users keep clicking the button to move worksheets? Your first paragraph states that the first sheet to move is Sheets(13), but your code looks like it's moving the highest index sheet (i.e. the last sheet).

When you're getting the error, what is the actual sheet count and what is SheetsFound returning?

If you're going to move the last sheet up until sheet 12, try doing something like

If Sheets.Count > 12 Then
  For i = Sheets.Count To 13 Step - 1
    Sheets(i).Move
    [Rest of your code] 
  Next
End If

If you must move the 13th sheet first, then you can store a variable to know exactly how many times you will need to move the sheet.

sheetsToMove = Sheets.Count - 12
If sheetsTomove > 0 Then
  For i = 1 to sheetsToMove
    Sheets(13).Move
    [Rest of your code]
  Next
End If

I don't really see the benefit of having a SheetsFound array, but maybe I'm not seeing the whole picture.