You can trigger it to switch after whatever number of numbers have been
typed in, for example:

Private Sub TextBox1_Change()
If Me.TextBox1.TextLength = 1 Then Exit Sub
If Me.TextBox1.TextLength = 2 Then Me.TextBox2.SetFocus
End Sub

Will switch after two numbers have been typed in. You can use this
logic to set it to whatever number you want by setting the TextLength =
?

Anita