No, for larger sites, you don't target only one keyword throughout the entire site. 40+ pages means you'll be covering a lot of ground on the entire subject.

I have a site with 150-200 pages that started from about 5. It started as a small made for adsense site, and then I decided to turn it into a large authority site. The homepage has always been targeting 3 main keywords.

Each page added either has a keyword in mind, or just a topic in mind. Sometimes, I'll see a great keyword and go after it with a new page or post. Other times, I'll just write a post about a topic, not even consider keyword targeting, and still get lots of traffic from long-tail searches.
I do try to fit in a link back to the home page with the anchor text as my main keyword.

The more focussed a site is, the better Google will like it (if other things are equal), but that's not the only factor!

It doesn't mean every article needs to be about exactly the same keyword, in fact, Google might penalise too much use of the same exact keywords. Google use the semantic context in their ranking algorithm of the text so the best thing you can do is also write naturally about related topics which are closely related and genuinely relevant to your main keyword. There will always be sub-topics meta-topics, and closely-related relevant topics, to whatever keyword you choose, so honest, relevant, natural writing about the subject the site is on, is the best approach.


Chris