The taxonomy plugin allows clients to add tags to a page. This is actually a link between two pages. There is no limit to the number of tags to a page.
Actually, many links are automatically created by the system. For example, when you create a page it is given a type and that's a tag managed by the taxonomy system. Some tags are managed by plugins in their own way (such as the page type) and others are viewed as generatic and the taxonomy system let you edit those.