40 HR per year: batter count / team count.

Click this link to view data and graph for seasons 1920 through 2012.

It contains a count of the number of batters who hit at least 40 home runs (HR) in a season and the number of American and National teams in that season.  The number of players is divided by the number of teams and represented as a percentage: the count of 40 HR batters relative to the number of teams.

I could have made this more meaningful if I had factored in the extra 5.2% games scheduled after expansion began in 1961/1962.  Using 38 or 39 HR for seasons before 1961 would have given more accurate numbers but I decided to keep it simple.

The graph really tells a story.  I was shocked that recent seasons were so much higher even when the increased number of teams was considered.

A record was set in 1961 (44%) that was not broken until the current record was set in 1996 (61%).  The 1961 number was also exceeded in these seasons: 1997, 1998, 1999, 2000.  2001 was close: 40%.  Since then:

2002 26.67%
2003 33.33%
2004 30.00%
2005 30.00%
2006 36.67%
2007 16.67%
2008 6.67%
2009 16.67%
2010 6.67%
2011 6.67%
2012 20.00%