Complex sentence: There have been 16 head coaches of the Boston Celtics, an American professional basketball team based in Boston, Massachusetts, playing in the Atlantic Division of the Eastern Conference in the National Basketball Association (NBA).
Split into simpler sentences: There have been 16 head coaches of the Boston Celtics franchise. The Celtics are an American professional basketball team based in Boston, Massachusetts, playing in the Atlantic Division of the Eastern Conference in the National Basketball Association (NBA).