“Seven and eleven are the magic numbers for the New York Giants today…With seven victories in their remaining 11 games before they close the 1936 campaign on Sept. 27, the New York team can win the pennant regardless of what the Cards did in their remaining 10 games. If the St. Louis club loses three games, the Giants can sew up the flag by taking 4 out of 11.” “Watching the Scoreboard.” (UP). Kirksville (MO) Express, September 17, 1936, p4.
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1947. “[The Yankees] reduced the magic number to four. That is the combination of games the Yanks must win or the Red Sox must lose in order to insure the flag for the Yankees” (The Washington Post, Sept. 12).