Currrently, Elvis Dumervil (Denver Broncos) and Jared Allen (Minnesota Vikings) are tied with 10.5 each.
Jared Allen
The linebacker with the most sacks in an NFL season is Justin Houston of the Kansas City Chiefs. He finished with 22.0 sacks in 2014.
That was 2004. He led the NFL in sacks that season.
There isn't really a record kept for fewest sacks in a season by a player, as there are thousands of NFL players who have not had any sacks. Michael Strahan holds the record for most sacks in a season, at 22 1/2.
The Dallas Cowboys led the league in sacks with 59.
In 1981 Klecko had 20.5 sacks and Gastineau had
Elvis Dumervil of the Denver Broncos. He had 17 sacks.
Through 12 games of the 2012 season that is DE Aldon Smith of the 49ers with 17.5 sacks.
Mike Berg of the Green Bay Packers had 9 sacks in the 1979-1980 season.
Unfortunately, Karras retired after the 1970 season. The NFL did not begin recording sacks officially until 1982.
In the 2007 NFL season, there were 1102 total sacks so the average team was sacked 2.2 times per game (34.4 times per season). In 2006, there were 1164 sacks averaging out to 2.3 sacks per game per team (36.4 sacks per season) and in 2005, there were 1182 sacks averaging out to 2.3 sacks per team per game (36.9 sacks per season).
5.5 in the regular season and .5 in the postseason.