Incheon free transit tour: no slots in online reservation for my tour choice. Can I still join on-site?

If the schedule for my particular choice of tour (slots show 15/15) is full in the reservation website, do I still have a chance to join that tour when registering on-site (off-line without reservation) as soon as possible when I arrive in Korea?

Do they only have 1 bus for that particular tour and how many passengers does it accommodate? I just want to know if there'll be more people given a chance aside from those people who reserved in advance. Because other tours seem short and not fit for my flight schedule, so I wanted to know if I'll still have the chance to visit that place e.g. tradition tour. Thank you!