A “magic number” is the combination of team wins and a contending team’s losses that will clinch a playoff spot or title. A “tragic number” is the opposite—a combination of team losses and a contending team’s wins that will eliminate a team. “Magic number” was cited in print on September 1, 1947, when it appeared in a baseball story by Associated Press Sports Writer Bob Grubb.
     
Both terms were used in the local baseball stories by the New York (NY) Post in 1986, when “magic number” was used for the contending Mets and “tragic number” was used for the non-contending Yankees.
   
 
Wikipedia: Magic number (sports)
In certain sports, a magic number is a number used to indicate how close a front-running team is to clinching a season title. It represents the total of additional wins by the front-running team or additional losses (or any combination thereof) by the rival team after which it is mathematically impossible for the rival team to capture the title in the remaining games. This assumes that each game results in a win or a loss, but not a tie. Teams other than the front-running team have what is called an elimination number(or “tragic number”) (often abbreviated E#). This number represents the number of wins by the leading team or losses by the trailing team which will eliminate the trailing team. The elimination number for the second place team is exactly the magic number for the leading team
 
(Oxford English Dictionary)
magic number  n.  (a) a figure regarded, in a particular context, as significant or momentous.  (b) Sport (esp. Baseball) the number which, at a given stage in the season, signifies the combination of wins for the first-placed team and defeats for the second-placed team which will assure the former of championship victory;
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1947 Gettysburg (Pennsylvania) Times 1 Sept. 4/3   So now the ‘magic number’ is 19, which means that any combination of Brooklyn wins and St. Louis losses totalling that number would see the Cards eliminated.
1989   P. Dickson Baseball Dict. 254/2   To determine the magic number, you combine the second place team’s wins and number of games remaining. From this total subtract the leading team’s number of wins. The difference plus one equals the magic number.
 
1 September 1947, Rockford (IL) Register-Republic, “Time Running Out on Cards as Dodgers Win Fourth in Row” by Bob Grubb (Associated Press Sports Writer), pg. 22, col. 1:
So now the “magic number” is 19, which means that any combination of Brooklyn wins and St. Louis losses totaling that number would see the Cards eliminated—by one game.
 
13 September 1947, New York (NY) Times, “Tigers Turn Back Yankees, 7-2” by Louis Effrat, pg. 15:
Before the game, the Yankees needed four victories to clinch the pennant, and though the finish found them on the wrong end of a 7-2 the magic number became three.
 
15 September 1947, New York (NY) Times, “Hatten Twice Tops Red, 13-2 and 6-3” by Roscoe McGowen, pg. 20:
The “magic” number ia seven—any combination of seven Brooklyn victories and St . Louis defeats mathematically will clinch the pennant.
 
26 September 1969, The Plain Dealer (Cleveland, OH), “The Sport Trail” by James E. Doyle, pg. 2-D, col. 1:
“Remember the good old late-season days,” says Berea Breezer, “when we used to be speaking of the Indians’ magic number? Theirs was a tragic number this year—and it came up months ago.”
 
18 September 1976, Saturday Times (Trenton, NJ), pg. D1, col. 3:
Magic Number: 14
Tragic Number: 14
(From a story about the Philadelphia Phillies.—ed.)
 
18 September 1976, The Morning Union (Springfield, MA), “Will Phils Fold?” by Dave Anderson, pg. 18, col. 1:
But who were those Phillies, and how did they turn the magic number into the tragic number?
 
32 August 1986, The Sunday Republican (Springfield, MA), “Baseball notebook” by Garry Brown, pg. C-2, col. 7:
And how about that nasty New York Post, which daily lists the Mets’ magic number and the Yankees’ “tragic number”?