A Gamemaster (GM) (also Storyteller) is a person who runs a larp. The number of GMs and their roles and responsibilities vary significantly depending on the type of larp.
In theatre-style larps
In a theatre-style larp, the GM is responsible for producing the game, arranging a venue and props, printing character sheets, and recruiting and casting players. At runtime, their role is largely answering player queries and dealing with rules questions. As a result, it is not unusual to have a 20-player game run by a single GM - though games with more complicated mechanics or frequent NPC roles will require more.