Most of our protocols close at ten travellers. People sometimes ask whether we would take twelve if two friends wanted to join. Usually we would not, and the reason is practical.
One vehicle, one table, one bath
Ten people and a leader fit a single minibus with luggage. They fit one long table in a farmhouse kitchen, which means one conversation at supper instead of three. In a Japanese inn, ten people can use a private bath in two turns of an hour; twelve need a third turn, and the last one runs late.
What changes above ten
Above ten, the leader spends the day counting. Every museum needs a group booking, every train needs reserved seats, and every walk waits for the slowest pair at more junctions. The protocol still works, but the day starts to belong to the logistics.
The exceptions
The Budapest and Pamukkale protocols take twelve, because the baths and sites there are large and the transport is a coach. A few routes stop at eight, where the inns are small. We print the number on every sheet so that it is never a surprise.