One of the mistakes people often make is trying to be all things to all people in their blog. Folks, there's a blog already out there on every topic you can imagine. From knitting blogs to squirrel awareness blogs to blogs about bubble gum. That's right, it's all been written about before. So how many topics should your blog cover? Well, if you're blogging for fun, cover as many as you want. Just do whatever makes you happy.
However, if you're blogging for fame and fortune, I would advise that you establish your "wheel house" and stick to it. In general, people start to return to blogs over and over because they know what it is going to be about. I know when I go to my favorite Yankees blog, there's not going to be something there about the Knicks. When I go to read a celebrity gossip blog, there isn't going to be in depth political coverage, etc.
So ask yourself, what made you want to start your blog in the first place? Whatever the answer is, that's what you should stick with.
Sure, there's room for branches out into other directions once in a while, but the reader should always be able to find their way back to your central theme. Also, it's kinda hard to keep all your "category buckets" fresh with new content. Once you have 20 different things to write about, eventually a category starts to get neglected.
Then again, you can throw all this out the window if you have multiple people writing for your blog. You can divide it up into sections or topics by author. Each author has a specialty of sorts, and sticks to that. I would avoid allowing all authors to write about all topics. You are likely to run into overlaps and conflicts. So just assign beats, and make sure everyone sticks to them.
There are some who disagree and think variety is better. I'm not against variety, but it can become a bit overwhelming. At the end of the day, I suppose the best advice would be to write about as many topics as you choose, as long as you can keep your site fresh and focused. If 25 topics is too much for you, no problem...scale it back.
Single topic blogs are also easier for start-ups. Establish your presence in a single topical space, then branch out after you get on your feet.