Some people go for multiple sites like 10 to 50 even hundreds. I personally think you need to master one site at a time, then either grow that one site or start another and master it. Once you can pump out 2 to 3 good sites that get traffic and earnings then start hammering out site after site.
If you are making $100 month profit from a single site you can build another one with a diferent nich and try to reach $100 monthly profit also.That will make you $200 per month.Than after you reach that goal build a 3 website and try again until you reach 20 websites and you are making at least $2000 month.