All Rec Royale Squads games start with 18 players, but in many of them once the game actually begins and players load into the gondola, that number will have decreased to 17-15 as a result of players crashing or leaving during the initial countdown. This guarantees that one or more teams will be left without a third teammate (and sometimes without two!).
In some rare but still experienced cases, players can join a game of Rec Royale and be placed as a spectator. This happens when they join the moment the game's startup countdown ends (as a result of people leaving during the startup countdown).
Extending the joining period to while the gondola's moving decreases likelihood of unfilled squads being placed at a disadvantage.