Any number of categories for the application to be placed in. There is no fixed list of categories - both the client and the web site will automatically show any categories that exist in any applications. However, if your metadata is intended for the main F-Droid repository, you should use one of the existing categories (look at the site/client), or discuss the proposal to add a new one.
Categories must be separated by a single comma character, ’,’. For backwards compatibility, F-Droid will use the first category given as <category> element for older clients to at least see one category.
This is converted to (<categories>
) in the public index file.