I think it's fine to have many topics on a single blog. I have for several years a few sites with many topics, traffic comes directly on the article pages, I have also categories pages and pages for every tag with more than 2-3 articles which helps internal linking of related topics pages. Of course it would be better to have a site devoted to one topic if you really want to target this topic, but then you're missing out on all other topics, it's a choice. What is better is to have at least a site/blog with many topics so you can spot in your stats a topic which attracts a lot of traffic and then you can open a site about it if you're really sure you want to invest significant amount of time to develop this new site.