I have one active website at the moment and I'm planning 3-4 more, but I also have two active blogs and planning the third...
I wouldn't recommend too many unless you don't plan to actively update most of them, or have several people workinbg together.
I have 2 main sites plus a bunch of casual sites. I'm still learning my ropes and I love doing business online!
I have ten blogs, only a couple of them are updated daily, the rest every three days, I am doing great and planning to fund my company next year to develop some serious websites.
i have 1 very active, and about 4-5 inactive [3 domain names are really good IMO, i just dont have time to update it
only have 1 active website with adsense.. but have tons of adult sites which i'm gradually beginning to sell and just concentrate on adsense and kontera suitable sites... though i'm doing this now to learn new coding and opensource software.. as i try to use different CMS or scripts for everysite... i actually enjoy building the sites.. and the rev i get is a big plus.
I have 4 web sites, of which 2 of them are monetized. One remaining is personal, and the other one is Lordi fan web site I have a domain, my-lg.com, there used to be LG Mobile portal there, but it no longer exists
I have around 20 sites at the moment but considering dropping most of them and leaving a couple as I do not have time maintaining all of them. And it is quite useless to have stale sites - they do not make any money and get devalued by google over time (their PR drops).