From:
http://www.sdabu.com/history_main.htm
"The size of umpiring staffs was also increased. The two-umpire system was the norm during the 1920s, but it became common practice to assign one of the reserve umpires to critical games or series; by 1933 three umpires were assigned routinely to regular-season games. The four-man crew was instituted in 1952. In the World Series the two-man crew, one umpire from each league, was used until 1908, when a pair of two-man teams alternated games. In the third game of the 1909 Series, all four umpires were on the field at the same time, thus establishing the four-umpire tradition that continued through 1946; in 1947 an "alternate" umpire from each league was stationed along a foul line in the outfield, thus creating the current six-umpire crew. Four umpires worked the All-Star Game from 1933 to 1948; the following year it conformed to the World Series format in putting the alternates on the field."