score:1
I had booked a 2 hour temple tour for my layover at Incheon in early January, 2018. They seem to have since updated the schedule, as I do not see that tour on their website anymore. When I arrived at the counter, I was told that my particular tour had been cancelled. I was lucky, though, and they still had one last free slot in a different tour.
The bus I took looked like this:
Every seat was occupied, so the capacity for spontaneous arrivals seems to be pretty limited. They also only had one bus and one guide for each tour. I do not think they can just get additional ones on short notice.
If you have the time and do not mind the immigration procedure and security check afterwards, you might go ahead and hope some people that registered do not show up. The tour guides and employees were very friendly, if a bit disorganized, and will probably do everything to somehow accommodate you. Do not expect to get into a tour if all the registered participants actually come, though.