Explanation of the novel's title

A magazine excerpt from the novel was originally published as Catch-18, but Heller's publisher requested that he change the title of the novel so it would not be confused with another recently published World War II novel, Leon Uris's Mila 18. The number 18 has special meaning in Judaism and was relevant to early drafts of the novel which had a somewhat greater Jewish emphasis.[3]

There was a suggestion for the title Catch-11, with the duplicated 1 in parallel to the repetition found in a number of character exchanges in the novel, but due to the release of the 1960 movie Ocean's Eleven this was also rejected. Catch-14 was also rejected apparently because the publisher did not feel that 14 was a "funny number". Catch-17 was also rejected so as not to be confused with the WWII film Stalag 17 So eventually the title came to be Catch-22, which, like 11, has a duplicated digit with the 2 also referring to a number of déjà vu like events common in the novel.

Source: Wikipedia