There are only two available multiplayer modes: Team Deathmatch and Battle Royale. Each mode can only have a maximum of 8 players. However, these modes can be played with lesser number of players if needed.
You’ll be assigned to one of the two teams to fight against each other. You can sortie with less than 8 players as long as the teams are fairly distributed. One main problem in this mode is that when a player leaves prematurely during the map, his/her teammates will be put to a significant disadvantage. Players cannot join to fill in an open slot while the match is in progress.
This is a free-for-all format where everyone is your enemy. It is more chaotic and will demand more situational awareness to avoid getting shot down while also trying to score yourself. Avoid entering this mode until you have good planes and you’re confident about your skills. Players can freely leave and join even if there’s a match in session.