The appendix or appendices of a thesis are a good place to put any data that does not comfortably fit into the body of the text. Tables, if they are larger than one or two pages, belong in appendices, as do long lists such as lists of interview subjects or archeological sites. Other types of appendices include source documents or translations. Mathematical proofs or the complete code of computer programs can also be presented as appendices if they are too long to fit into the main flow of the text.
A thesis can have a large number of appendices. As a rule, each data set left out of the body of the thesis should have its own appendix. Giving each data set its own appendix makes it easier to use them for reference purposes. The appendices should be presented in the order in which their data is used in the text, unless there is some other inherent way of ordering them. For instance, a set of translated documents could be presented in chronological order or sorted by author.
Every university or department has its own specific requirements for the formatting of appendices, and these should always be followed carefully. What they tend to have in common is that appendices should consist primarily of data, with a minimum of argument. Writing which makes a new claim or which advances the argument of the thesis belongs in the main body rather than an appendix. Most universities do not count appendices when considering the word count of a thesis.
Not all appendices are simply tables or lists of data. Increasingly, appendices are being presented in alternative formats. Sound or video recordings can be presented in digital format, as can working copies of computer programs. Maps, diagrams or illustrations may need to be included on sheets that don't fit the binding of the thesis. In all cases where the appendix is of an unusual format, it is best to consult the department or university's thesis guidelines.