Number of Pandemic players?
When the game is started the roles are shuffled together and each player is dealt a random role. Having more roles then players allows for some greater variations in the game as you will not always get the same roles in every game which can change how you play the game. You can see in the rules how it deals with the extra roles. Also it should be noted that in the current edition there are 7 roles for 4 players in the base game.