Well stick to a single niche but try to cover as much topics inside that niche. For instance, if you were running a blog on "how to make money online" you would definitely want to cover topics like "how to create a website" or "what is the best CPA network". So in this way, you would definitely want to run a Multi-topic blog.
However, if by "topic" you mean a niche, then you don't want to mix too many different niches. Cooking doesn't go well with technology news.
So right in the middle of these 2 opposite meanings of the word "topic" is your golden middle where you want your blog to be.
Short & practical version:
1. Pick a main keyword (preferably a 10k+ exact searches keyword)
You cover everything that can be considered a sub-niche for that keyword.
2. Go to google keyword tool, enter your main keyword, check "only related ideas", more than 100 global monthly searches, less than 10k globalb monthly searches and hit search
3. Go through the list of keyword ideas and ad group ideas and find keywords that can you can write an article about
4. You now have a long list of topics to cover and all of them will be relevant to your audience