In academic English, the preferred plural of incunabulum is incunabula; outside fairly academic English, no-one uses the word at all. (It means a book printed before 1500 CE.)
Incunabulum was originally a Latin noun, of a type ending in -um whose regular plural ends in -a (see -um in Latin). It originally meant 'swaddling clothes, wrappings for a new-born baby', and came to be used figuratively for 'the beginning' of something, and in the history of printing, the first productions, or 'books from the cradle of printing'.
There was an English form incunable used in the 19th century, but it is little used now. However, its plural form is easier: incunables.