There are a number of approaches.  All with different drawbacks.  If you
want to prevent a user from ever entering anything other than numbers (or
other special chars) with the keyboard, you'd want to use the KeyPress
events.  Something like this:

  Private Sub TextBox1_KeyPress(ByVal KeyAscii As MSForms.ReturnInteger)
     Debug.Print KeyAscii
     Select Case KeyAscii
        Case 48 To 57
           ' Asc("0") to Asc("9") - these are okay
        Case Else
           KeyAscii = 0
     End Select
  End Sub

That said, you're still presented with the issue of the user pasting other
text into the textbox.  It's up to you how to handle that, of course, but
here's one approach to validate every time the textbox content changes:

  Private Sub TextBox1_Change()
     Static PreservedText As String
     Dim ThisChar As Long
     Dim i As Long
     ' Danged users, gotta check to see if they
     ' pasted something other than our valid chars
     ' into the textbox!
     With TextBox1
        For i = 1 To Len(.Text)
           ThisChar = Asc(Mid$(.Text, i, 1))
           If ThisChar < 48 Or ThisChar > 57 Then
              ' Illegal input, revert!
              .Text = PreservedText
              Exit For
           End If
        Next i
        ' Save copy of new textbox contents.
        PreservedText = .Text
     End With
  End Sub

Later...   Karl