I haven't done rigorous testing but this should get you started
Code:
Sub TestTables()
Dim i As Integer, x As Integer, iTbl As Integer, sOthers As String
For i = 1 To ActiveDocument.Tables.Count
If ActiveDocument.Tables(i).Columns.Count > x Then
x = ActiveDocument.Tables(i).Columns.Count
iTbl = i
sOthers = ""
ElseIf ActiveDocument.Tables(i).Columns.Count = x Then
sOthers = sOthers & i & ", "
End If
Next i
If sOthers <> "" Then sOthers = "Other tables with same column count are: " & sOthers
MsgBox "The table with the most columns is " & iTbl & vbCr & sOthers, , x & " Columns"
End Sub