I have got 3 websites now and one more I'm going to start working on in near future. Each of them is important for me, one is kinda blog and portolio, the other two are about stuff I'm interested in. I can't imagine having more than 10 serious projects, only if these are simple blogs or affiliate/adsense sites it that don't matter so much and just bring you some extra cash from ads. I used to work seriously on my own projects, and maybe that's why 2 of them are still under development, he-he...
I have 25 blogs. It's not hard to manage them but it all depends on their purpose. Right now I'm an Adsense blogger and soon I will add affiliate marketing. My goal is to have 100 blogs in 2 years earning $200/day. Inspirational Note: "Some days I don't work on my blogs but the checks keep coming in."
Take a lesson from a veteran. I had 49 sites at once, and then when I moved into BH I had over 20k domains. After I moved out of BH I was back down around 50 sites. The sites made beer money, or a couple hundred a month. One day I decided I had to many sites and was not able to put as much value into each site as I should, so I closed, or sold all the sites till I got down to 3 sites. Since that day I have only allowed a new site into the mix if all my other sites have reached a $300k+ income marker, or have hit a good cash point and I need to change my focus for a while for some R&R from the one site. My one main rule is never go past 5 sites (I normally only have 2-3, at 1 launched per year). If I have 5 sites and there is a 6th site that I want to build, it has to look better than one of the 5 I already have, and I have to sell one of them five to start the new one. The first year was hard when I moved into this rule, I dropped off sites as my core sites did or did not grow. As time moved on, one site would go from beer money each month, or a couple hundred, to thousands. After about two years one of them sites cracked 300k, then 3 years later it moved over 600k, the other two sites followed. I was focused on my goals and was able to move the sites better then I did before. Last year I sold one of them sites to a VC, and this year we are in the process of selling another site to a VC, while we bring on two other sites that I think will do even better them sites. If you can only put 10% of your time into each of your sites your only going to get 1% of the value returned for your work.
I have around 15-20 sites, it's not about the amount of sites you have. The more sites you have the harder they are to maintain well. If you build a good, strong site initially then it can be as successful as 20 smaller ones.
I currently own 50 websites, and trying to get rid of them since I'm starting a local business. But I will keep about 10 that are earning automatically with zero maintenance. I will be posting a few sites from sale here soon..