As I mentioned a while back, there is theoretically no limit to how many contacts an app can get simultaneous input for. The official number is "dozens and dozens", but internally we use 52 to set performance goals for the platform. In choosing the number to set as our arbitrary v1 benchmark, the team evaluated a number of application scenarios and determined that games were the category of apps most likely to need the highest number of simultaneous contacts. After looking into a number of game concepts, it was decided that supporting a game involving 4 people simultaneously using 10 fingers + 3 game pieces (4*(10+3)=52) would go far beyond the needs of most applications we foresee for v1. Over time, as Surface takes on new form factors and new app concepts are dreamed up, this benchmark will surely increase.
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