I'm a freelance writer and in the past I have worked with a lot of website owners, some of which have 10+ websites on the go. Personally I have control of two sites right now and I feel this is the perfect amount to manage. I guess if you have the capital to outsource your work like my clients do then it will be easy to manage such a vast amount of sites.
How many sites do you have at once, and what do you think is the sweet spot? I know a lot of people have a blog network, which helps with ranking, but personally I like to focus on just one thing at once.