Group charter starts at twenty, and that is a real rule.
Conferences, incentives, roadshows and site visits work better as one departure and one arrival than as three scheduled flights with three chances of a delay. The schedule follows your agenda rather than an airline's.
The threshold is not a marketing convention. Under EASA rules an aircraft seating more than 19 must carry at least one cabin crew member, and the FAA uses the same number. Below 19 you are looking at a private jet instead.
