appendix / appendices / appendixes
The plural of appendix in a book and in anatomy.
Both appendixes and appendices are standard. Prefer appendixes for the extra sections of a book and for the organ. Appendices is also correct. Pick one form in a document and keep it.
Examples
UseSee Appendixes A and B.
UseThe surgeon removed two appendixes.
Avoidmixing appendixes and appendices for the same book
Neither plural is a mistake. The above is our guidance.