I run only one startup: www.postjobfree.com
I think it's better to focus on one thing that do many things at the same time.
But even with one web site there is problem with focus: there are so many features that I'd like to incorporate and so few hours in a day to actually implement them.
PostJobFree generates $20K+/month revenue (gross, not net) and allows me to work full time on it.