We get asked this question all the time. The answer is - it depends. A small business start-up site can usually be found with 5-8 web pages. That would be a nice start. However, as the business grows, so should the website - and that small 8-page site can grow to 18 or 28 pages.
The point is - a website should contain enough information to describe the company, the company products and services, one or two "value-added" pages of information, along with a easy-to-use contact page. In other words, to build a site with an large number of pages, just because it looks good or someone said you should have more pages, is over-bloating the site of the site and cost of development.
Keep it simple, informative and to-the-point. Anything else is extraneous and not cost-efficient.