What is the minimum number of teams required for a tournament to be an officially sanctioned by Vex
16 teams need to be registered. Whether you can count registered teams who then fail to turn up is apparently region dependent for some reason. (The link talks about skills events, but the underlying rules are the same so the answer is probably the same in both cases).
Thank you, Oliver W. We are just getting into the higher levels of competition with our robots.
There are other requirements to be an official qualifying event for this season, described here.
One is that the event needed to be posted on robotevents.com prior to 2 November 2015, so it’s no longer possible to organise an event that would qualify teams for this season’s state champs or world champs.