Modern era — 17, by the 1953 Boston Red Sox against Detroit.
All-time — 18, by the Chicago White Stockings (now Cubs), sometime in the 18xx's.
Let me go rummage some details….
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Boston's big inning: 18-June-1953, bottom of the seventh against Detroit. The Red Sox won, 23-3. Box not available, but here's the line scores of the day: http://www.retrosheet.org/boxesetc/1953/06181953.htm
Modern era — 17, by the 1953 Boston Red Sox against Detroit.
All-time — 18, by the Chicago White Stockings (now Cubs), sometime in the 18xx's.
Let me go rummage some details….
—–
Boston's big inning: 18-June-1953, bottom of the seventh against Detroit. The Red Sox won, 23-3. Box not available, but here's the line scores of the day: http://www.retrosheet.org/boxesetc/1953/06181953.htm
Chicago: 06-Sept-1883, also bottom of the seventh, also against Detroit (Wolverines). Won the game, 26-6. Not even line scores, but here's the day: http://www.retrosheet.org/boxesetc/1883/09061883.htm
Back in 1994 the Texas Rangers set a Major-League half inning record of 16 runs.
That same year the Montreal Expos set a new National League half inning record of 13 runs.