From my experience, working on as fewer sites possible the better. I used to create and work on 7 niche sites at once, it was a nightmare to do linking with 7 sites, along with 7 different other things, like 7 emails, 7 CMS/FTP logins, 7 sites to advertise etc..
Then a few years ago I abandoned those 7 sites and now all I work on is just one site, and I have to tell you, it is more efficient to work on just one site. I am building traffic up like mad and it is a lot less stressful. I am able to increase this one site's DA and PR so much faster than worrying about 7 sites at once.
So that is my humble opinion and experience on the matter.