One of the dev's of Left 4 Dead 2 was so into this "number 42" that there are references to it it all throughout the game.
Example: 42 steps in staircases, 42 painted on vehicles, 42 palings in a line of fence, 42 zombies in a superhorde, 42 windows in a building etc...
Probably the most famous reference to "42" in Left 4 Dead 2 is during the campaign "Dark Carnival."
In this campaign there is a minigame called "The Shooting Gallery" where a player needs to hit or shoot targets -that pop up from holes - to earn points. Once you earn 42 points, it triggers a special audio cue: the Jockey's maniacal laugh. If you do this for the
first time you get an achievement and the machine spits out hundreds of carnival ride tickets.
This is a direct reference to Douglas Adams' "The Hitchhikers Guide To The Galaxy!"
There's also another minigame in the same campaign where you shoot 42 moving targets in a "Shooting Gallery" to acquire a "Gnome" that you must carry to the end of the campaign and get on the chopper in the finale whilst carrying it.
Doing it for the
first time collects an achievement.