It's hard to nail down the magic number of pages for a site to be effective. Google's Matt Cutts has said that a large number of pages doesn't always affect your SEO ranking. But he's also said that the more pages a site has, the more opportunity there will be to use great keywords and other SEO strategies. So essentially, bulking up your site with lots of extra pages won't do you any good if they aren't great pages. In today's SEO world, the right amount of content on a site that has the correct technical architecture to align with the search algorithms rules.
Ranking Does Require Some Content
If you have a ton of pages on your site, the only way to get a good ranking is to ensure that they are optimized perfectly and get at least some traffic each. Using lots of keywords effectively will help. But depending on your niche, you may not have enough creative original content to fill up dozens of pages. A smaller site isn't always at a disadvantage. Create a natural site and optimize the pages and the technical structure to a very high standard. Successful SEO is absolutely quality over quantity.
After all, the goal is to get customers to your page and get them to act. If they can choose between 15 spammy, boring pages on a site or 5 well-executed, informative and interesting pages, guess which one they will choose. The number one rule of SEO is to remember that you are designing for people, not bots. It doesn't matter how many people you get to your site if you can’t get them to follow through with your call to action.