Players can choose to play as one of nine character classes in these teams, each with his own unique strengths, weaknesses, and weapons. In order to accomplish objectives efficiently, a balance of these classes is required due to how these strengths and weaknesses interact with each other in a team-based environment. Although the abilities of a number of classes have changed from earlier "Team Fortress" incarnations, the basic elements of each class have remained, that being one primary weapon, one secondary weapon, and one melee weapon. The game was released with six official maps, although over 100 maps have since been included in subsequent updates, including competitive remakes of older maps and community-assembled maps. When players choose a gamemode for the first time, an introductory video shows how to complete its objectives. During matches, the "Administrator", a woman voiced by Ellen McLain, announces various game events over loudspeakers. The player limit is 16 on the Xbox 360 and PlayStation 3. On the PC, in 2008 Valve updated the game to include a server variable that allows up to 32 players. "Team Fortress 2" is the first of Valve's multiplayer games to provide detailed statistics for individual players. They include: time spent playing as each class, most points obtained, most captures or objectives achieved in a single life, and many other things. Persistent statistics tell the player how he or she is improving in relation to these statistics, such as if a player comes close to his or her record for the damage inflicted in a round. "Team Fortress 2" also features numerous "achievements" for carrying out certain tasks, such as scoring a certain number of kills or completing a round within a certain time. New sets of class-specific achievements have been added in updates, which add new abilities and weapons to each class once unlocked by the player. This unlockable system has since been expanded into a random-chance system, where the player can also obtain the items simply by playing the game. "Team Fortress 2" contains five core game modes. There are several alternative game modes in "Team Fortress 2". These modes consist of a small amount of maps and detach from the core game modes in some way. Mann vs Machine (MvM) is a cooperative game mode where players must defend their base from waves of robots modelled after all the nine playable classes, and slow-moving tanks carrying bombs. Robots and tanks drop credits upon their death, which players can use to buy upgrades for themselves or their weapons. The players win upon successfully defending their base from the bomb until the last wave.