Start one site - spend a few weeks or months filling it up with content, and creating content to be automatically published at later dates. You want at least a 3-6 month backlog of content for auto-release. With some exceptions, most sites are like wine and need to age and do their thing. After you've got your backlog set up, you can move onto another website and repeat. Eventually, loop back around to see where the first website stands, analyize, make adjustments, corrections, new content - repeat. If its your very first website, spend more time on the first one than you think you should.