He changed the title to Catch-22 after another novel with a similar number in the title, "Catch-18," was published before his book.
There's a fun story behind this. Heller reportedly chose the title "Catch-18" because he thought it was a good, attention-grabbing title. He even went so far as to design the book jacket with the title "Catch-18." However, he found out that a book by the same name had been published by another author.
To avoid confusion, Heller changed the title to "Catch-22," which he thought still had the same ironic and absurd feel. He didn't want to change the number altogether, so he went with a different number that was just as nonsensical and symbolic.