It could very well have been chosen at random, but it's just as likely that he simply picked it up as the go-to "silly arbitrary number" from Lewis Carroll, who

It could very well have been chosen at random, but it's just as likely that he simply picked it up as the go-to "silly arbitrary number" from Lewis Carroll, who used it multiple times. "Rule 42" in the court, "Rule 42 of the Code" when hunting the Snark.