Sometimes the words "template" and "theme" are used interchangeably since they both refer to a system for efficiently managing the design and content of multiple web pages. A theme refers to the design components of a page (layout, typography styles, management of content types) as well as how content is managed on that page. So a template can be thought of as a subset of a?theme.
Try to identify how many Themes your site will have i.e. how many different design layouts and templates.