A template file manages and displays multiple pages. This allows for making changes easily to many pages. The different components of a template file include the display of specific content types as well as a unique design. The template design is also called a "theme" in Drupal. Unlike print design, where each page in a brochure can have a different layout and still take the same amount of time to print, the more web templates there are, the more time it will take to develop the site. The advantages of a Template include: More efficient code and less time to code a site resulting in a less expensive website, easier maintenance of a website since changes can be made globally, improved usability since it’s easier to view pages that have common layouts and navigation schemes, easier display of content from a database and more efficient integration with Responsive Web Designs (mobile friendly).
Try to identify how many Templates you think your site will need e.g. Home page, Blog overview, multimedia gallery etc. The fewer templates needed, the less time and cost there will be to build the site.