如何在列表框中限制最大数量的值(条目)?

I currently have two ListBoxes (ListBox1 and ListBox2) with 12 static values in ListBox1 (value1, value2, value3, etc.) that allows the user to transfer values between them using an add and remove button. I also have a drop down box. How do I enforce a max on ListBox2 when a certain selection is made on that drop down box? In other words, if I just wanted to allow a max of one entry from Listbox1 to be moved to Listbox2 when a value is selected in the drop down box.I currently have two ListBoxes (ListBox1 and Li