I guess "many" is a relative term, but given the rarity
of the name, it's surprising how many there are. I checked the Dayton
and Cincinnati, Ohio phone books (I work at a library),
and there are 15 in the Dayton phone book (including
me) and 12 in the Cincinnati phone book (including my dad
and his cousins). My dad and my aunt and I just re-discovered
some cousins, and they said there's a Veterans of
Foreign Wars post in Fort Recovery, Ohio, named after
a Mannix. Our branch probably came from that area, but
I need to check some other sources to make sure.

I am starting to research the Mannixes, but just in the USA.
My mom's family has been much easier to find -- they were mostly
German Catholics and English Quakers. Odd combination.