How many players from each team are on the court in indoor volleyball?

How to get close to this number

Work from the court. It is a small rectangle by team-sport standards, and the players have to cover it without colliding while the ball is in flight for a couple of seconds at a time. Six is what fits — three across the front and three across the back is the only arrangement that covers the area cleanly.

The rotation rule is what makes the number structural rather than arbitrary. Players rotate clockwise each time the team regains serve, which means every player must be able to play every position over a set. A sport with mandatory rotation cannot have specialists in the way other team sports do, and the count and the rule are inseparable.

The beach comparison in the explanation is the neat proof: the outdoor version plays two a side on a smaller court, with no rotation because there is nowhere else to stand. Halve the court and the team shrinks by two thirds, which shows how tightly team size is bound to playing area rather than to the rules of the game itself.