There is no user limit since v19.0 so you could get away with simply making 20 <a target="_blank" href="https://www.papercut.com/support/resources/manuals/ng-mf/applicationserver/topics/user-guest-internal.html" rel="noreferrer noopener">internal users</a> (or 10 if you use both card fields) and set the ID fields to the card number and disable the internal users so they can't log in