Focus on a single blog because it is more ideal, practical and efficient this way. First and foremost, it would be easier to promote a single blog compared to promoting multiple ones. Imagine how hard it would be to send your eight blog links to other people. This is not ideal because people will lose interest immediately with your promotion because it would be wasteful of their time and yours. Secondly, using a single blog allows you to focus on specific criteria you like and not just be forced with other ideas. If you give concentration and passion on one thing, it will grow efficiently and effectively. Lastly, I believe that the cost of domain hosting is quite expensive even for one blog site. What more if you host 4-5 blog sites simultaneously? Having multiple blog site is exciting yes but hold your horses for a while. Wait when your blog site gets adequate recognition and popularity, then that time would be the right moment for you to add an additional blog site.