The Main Thing - in my opinion, for all tournaments with an unpredictable random number of participants - the first group stage should convert the random number into two brackets - winners and losers. In our case, probably - 4 teams in the winners bracket, 4 in the losers bracket.
This task is not solved by a rigid setup of groups of 4 participants. For example: 12 participants. 3 groups of 4 participants. The output gives us 3 first places, 3 second places. And we get "who knows" how to fit this further into a playoff scheme without additional indicators.
This task is solved by a rigid setup of the number of groups in the first stage being a multiple of 2. From which the first places advance to the winners bracket, the second places to the losers bracket.
Groups. The simpler the better. Groups of 3 participants and groups of 4 participants are both suitable for achieving the goal.
Groups of 3 participants. It's simple. 3 rounds. In each round, only two participants play against each other, one rests. The intensity is reduced for both judges and teams, which is good.
1st place - winners bracket. 2nd place - losers bracket.
Groups of 4 participants. The double-elimination scheme, which is used everywhere in esports, and finally also on HeroesWorld - in the Final Battle. It is also played over three rounds. A total of 5 games.
Round 1. Match 1 and Match 2 (initial draw).
Round 2. Match 3 - "Winners" - winners of matches 1 and 2. Match 4 - "Losers" - losers of matches 1 and 2.
Round 3. Match 5 - "Decider" - loser of match 3 vs winner of match 4.
The winner of Match 3 advances from 1st place (to the winners bracket).
The winner of Match 5 advances from 2nd place (to the losers bracket).
Again, in round 3 the intensity decreases - only one match is played, not two. One participant has already secured first place and rests, one has already been eliminated.
Classic round-robin groups, each against each (whether 4 participants or more) - regularly look questionable, as there are constantly meaningless games in the last rounds or a chance for collusion. But in this scheme, there are no meaningless games at all.
(clarification - I have nothing against large round-robin groups on HeroesWorld in Novice or Favorites, where one of the tournament's goals is to let everyone play more - that is a special case and is not relevant here).
Since we have a team tournament, and teams can tie with each other - one nuance - in any case of equal points and the need to somehow choose which team advances - the solution is ONE decisive game between the strongest players of the teams (to even theoretically eliminate the possibility of the decisive game being played by the strongest player instead of another).
Any number of participants (teams) can be converted into groups of 3-4 participants. And from the group stage, proceed to the playoff stage with double elimination.
If we have 12 teams - that means 4 groups of 3.
If we have 15 - that means 4 groups of 4+4+4+3.
27 teams? 8 groups of 4+4+4+3+3+3+3+3.
And playoffs with 8 winners, 8 losers.
A unified solution for any number of participants. 3 rounds of group stage and double-elimination playoffs.
Although maybe it won't suit Arma at all - double elimination might be too slow for this tournament?