As in “Appendix of a book”?
appendices
appendixes or appendices, take your pick .
Either “appendixes” or “appendices” are correct, I believe for all meanings of the word. Dictionaries will give you which is preferable for which use, I think – my guess is that the more Latinate “appendices” is slightly favored for supplemental material appended to a book, contract, etc.
What? You couldn’t Google this yourself?
Hit wrong repy button : sorry. Should have repied to OP.
I was taught to use “appendices” for the book-part meaning and “appendixes” for the body-part meaning (though things may have changed since then).
How often do you use a plural for a body part that folks only have one of?
Tris
Well, I’ve removed many appendices. 
I just had the pleasure of perusing the APA Style Guide in some detail, and they prefer Appendixes for the various sections added on to the end of a research paper. That surprised me. In any case, my paper only had one appendix.
Ooh! Enjoying those serial commas before “and”? Or the capital letter after a colon? Good times. [/APA Style hijack]
appenda is inaccurate, but certainly looks the part.
{Bolding mine}
In this case, is would be correct as you’re talking about a word itself instead of the items the word identifies. 
When I opened the thread I was wondering under what circumstance would one need to use such a form of the word as there is one per body. Now I know.
And I didn’t think of books.
If the plural of appendix is appendices and the plural of index is indices, can I use ibices as the plural of ibex?
If I have more than one high explosive, can I use semtices?
Are more than one set of overpriced sunglasses Uvices?
The plural of pic is pix, so we might as well use appendix as the plural and change the singular to appendic. This “back formation” has worked before for many successful words.
Dunno about those, but in the realm of operating systems, there are several Unices (Unix variants.)
It’s one of a series of words anglicized.
Appendois, I believe.
I’m pretty sure that “series” is both the singular and the plural, in both English and Latin. Third declension Latin nouns are tricky: The plurals are fairly consistent (root of the word plus -es), but the nominative singular can be anything at all. The most common is -is, -es (such as penis, penes or testis, testes), but you can also get -es in both cases (series), or something where the singular doesn’t contain the full root (index, indices and appendix, appendices), or even something with no particular resemblance at all (the plural of “Jupiter”, if one ever had occasion to use such, would be “Joves”)