SEO, which is short for Search Engine Optimisation, works by making your site "friendlier" for search engines such as Google to list in their index. No-one knows for sure just how many pages Google has indexed. It gave up posting that figure some time ago as it's irrelevant to the average internet surfer - we only want relevant results when we're searching and don't really care about the literally millions of pages that were discarded on the way.