This is something that I've been thinking about for a while. I have visited the websites of several SEO companies and many of them have a small to medium sized staff presumably working with a good handful of client websites. In my opinion, there is simply not enough time in the day to effectively rank all of your client's websites using traditional SEO. As in, all of the conventional white hat SEO theory pertaining to 'creating great content', obtaining high authority links, guest blogging etc would take a millenia to rank numerous websites and would surely require a very large staff to keep on top of this stuff.
So.. what do you guys think? How do they do it? What do they do? Outsource to Fiverr? ODesk? This is something I am very curious to know.. how these small companies can possibly find the time to operate effectively using only professional white hat SEO best practices (or so they say).