While SEO becomes more and more important, people try to find angles that they can exploit or take advantage of so that they can achieve good rankings. In one specific instance, High Rankings forum post asks if there's a minimum number of pages that need to be targeted to achieve decent rankings.
It's important to note that in general, a site as a whole is collectively not ranked. Each page ranks individually. Therefore, you need as many pages as possible to say what you need to say in a manner that is understandable by your visitors.
Of course, this means doing keyword research and then writing content in such a way that benefits the visitors in as many pages (or as few pages) as possible. There's no set number.
But what about theming? Does Google look at similar pages and throw a site in a single category? It's an ongoing debate, where some people say that Google does this, while others say that they don't.
In general, it's agreed upon by all to perform diligent keyword research and to write from the start with keyword phrases in mind.