Players with 5 or more seasons with at least 40 HR:
| Player | 40 HR Seasons |
| Babe Ruth | 11 |
| Alex Rodriguez | 8 |
| Barry Bonds | 8 |
| Hank Aaron | 8 |
| Harmon Killebrew | 8 |
| Ken Griffey | 7 |
| Sammy Sosa | 7 |
| Adam Dunn | 6 |
| Albert Pujols | 6 |
| Jim Thome | 6 |
| Mark McGwire | 6 |
| Willie Mays | 6 |
| Duke Snider | 5 |
| Ernie Banks | 5 |
| Frank Thomas | 5 |
| Jimmie Foxx | 5 |
| Juan Gonzalez | 5 |
| Lou Gehrig | 5 |
| Manny Ramirez | 5 |
| Ralph Kiner | 5 |
From this list, Duke Snider (407), Juan Gonzalez (434), Lou Gehrig (493), and Ralph Kiner (369) are the only players to not finish their careers with at least 500 career HR. Albert Pujols is currently 8 away, and Adam Dunn 70 away.
Eddie Murray is the only member of the 500 Home Run club that did not have at least one 40 HR season in his career. The most HR he ever hit in a season was 33 in 1983.
Other Facts:
The first time a player had 40 or more HR in a season was in 1920, when Babe Ruth hit 54. In fact, out of the 13 instances where a player had 40 or more HR in a season in the 1920s, 8 of them were by Babe Ruth.
Total 40 HR seasons by decade:
1920s: 13
1930s: 18
1940s: 9
1950s: 34
1960s: 34
1970s: 20
1980s: 13
1990s: 72
2000s: 87
2010-2012: 10
There were more occurrences of a 40 or more Home Run season in the 20 seasons between 1990 and 2009 (159) than all previous years before 1990 combined (141).
Single Season:
17 players had at least 40 HR in 1996, which is the most of any season ever.
The season outside of the 1990-2009 time frame with the most occurrences of players having at least 40 or more HR was in 1961 (8 players).
Age:
The youngest player to ever hit 40 or more HR in a season was Mel Ott in 1929 (20).
The oldest players to ever hit 40 or more HR in a season were Hank Aaron in 1973 and Barry Bonds in 2004 (39).
Team:
The New York Yankees have had more players hit 40 or more HR in a season than any other team, with 30. 2nd is the Chicago Cubs, with 19.
Games:
The least amount of Games played in a season where a player finished with 40 or more HR was 111 and 112 respectively by Ken Griffey Jr and Matt Williams, both doing so in the strike-shortened 1994 season.
Plate Appearances:
The least amount of PAs a player finished a season with when hitting 40 or more Homers was 465 by Hank Aaron in 1973.
At Bats:
In 2004, Barry Bonds hit 45 HR in 373 AB, the least amount a player has ever had in a season where 40 or more HR were hit. This can mostly be attributed to his 232 BB, 120 of which were intentional passes.
Runs:
In 1997, Mark McGwire hit 58 HR, while scoring 86 Runs. This means that over 2/3 of his Runs that season came from batting himself in.
Adam Dunn:
Adam Dunn has hit at least 40 HR in a season 6 times, but his 2012 season, one in which he had 41 HR, is the strangest of all 310 instances.
In 2012, he only had 110 Hits, which is the lowest amount a player has ever had in a season where he hit at least 40 HR.
His 222 Strikeouts in 2012 is 2nd only to Mark Reynolds' 223 in 2009 (Reynolds had 44 HR in 2009).
His .204 BA in 2012 was by far the lowest (2nd lowest was Curtis Granderson's .232 that same year).
His .468 SLG in 2012 was also the lowest of any season where a player hit at least 40 or more HR. In fact, there have only been 3 seasons where a player hit at least 40 HR and did not have a Slugging % higher than .500 (Adam Dunn 2012: .468, Adam Dunn 2006: .490, and Curtis Granderson 2012: .492).
Finally, his .800 OPS in 2012 was also the lowest of all 310 times there has ever been a season where a player hit at least 40 HR.
Source: baseball-reference.com
