Well see, how does "how do I display x num components after an event" relate to "Can you group textboxes?" ?
Create a JPanel that is to hold these "textboxes" and place it on the GUI at the beginning.
Write a selection listener, and add it to the listbox, that then removes all components from that Panel and adds the "proper number" of text boxes to it, then calls validate() and/or repaint() on that panel.
no FW, sure by using validate() for addOns/removes JComponents on most cases works, but correct way is revalidate()...(revalidate covered validate too), in proofApp's you have to be care and hold this rule, hmmm for example some JComponents doesn't works correctly and as you expected (JComboBox missing data from custom JList ....) and other nice stars on this heavens
if you remove or add new JComponents to the Container
if you want to modified Container or its Components, and if Container and its JComponents are placed by using some LayoutManager, and this Container was once time visible then
validate();// in this case revalidate() too
repaint(); // maybe doesn't need in all cases, but just in case