From 3.1.2. Preliminary Rounds Seating it could be interpreted via method of elimination that the smallest number the rules account is 4. Actual sanctioning people should confirm or deny that at their leisure.
> http://www.vekn.net/tournament-rules wrote:
> ...The judge will randomly and impartially assign players in groups of five. In the event that the number of players is not evenly divisible by five, the judge will assign players in groups of four and five, such that as many groups as possible contain five players.
>
> If the players cannot be divided into groups of 4 and 5 (i.e., there are 6, 7, or 11 players), then the judge may elect to use a seating arrangement in which some players sit out each round (while still ensuring that every player plays the same number of games, usually equal to one less than the number of rounds). ...
>
Configurations for seatings of three, two or one player are not laid out, whereas 4, 5, 6, 7, .... etc. are. To be sanctioned it would be logical you need to first seat people via the official tournament rules.
In theory smallest sanctioned Storyline event could be 1 as there could be a event deck with pre-arranged play order that has it's prey control it as it plays with open hand and uses cards as it can while taking certain actions with first priority. Thanks to being able to bend the rules in storylines, sky is the limit and one is the floor, altho if there's a storyline deck that the judge controls, and he does not take part in the event, with rules where the storyline deck auto wins unless anyone of the players does the floor become zero players? If the turney by rules happen but nobody but the officials take part on it, is it a tournament?
Trust in Jan Pieterzoon.