I just came to see if anyone has the answer to this question, but having looked at the rules on page 5 it makes sense. With 2 players, only a single climber can stop on a space. With 3 or 4, it's two climbers. with 5 players, it's also two climbers, but you also have the choice to include the 'red piece' making it three climbers on a space.
The red piece is marked on the board.
This means that when you play in 5, if you choose to play with "Red piece" rule, above 7000 mt every space can only have 2 climbers instead of 3 and above 8000 mt, just 1 climber instead of 2.