No generic tickets. Last year each table had a sign showing full or not full (or something like that). You just approached the person running the game and asked if there was an available seat so that you could play. Most people running games are accommodating. Just a few are not.
For almost all of my events I specifically state I leave at least 1 seat reserved for walkups. So if I think I can handle 8 seats I only put 7 in the system for pre-registration.