Categories that were designed with the best of intentions at the beginning of a project can go horribly askew. Do yours still make sense for what you are doing? You should be able to figure this out after the first month or so of continuous effort, and go back and re-edit to put things where they belong.
Take the approach that each category needs to be thorough enough to capture everything that you have written on a broad topic, but not so comprehensive that every narrow topic gets its own category.
How many categories do you need? As a rule of thumb, plan to have at least one post per month for each category that is in season. A cooking weblog with a "Juneberries" category can be excused to only have three annual posts - one planning the season, one describing what happened, and one in January remarking on eating something preserved for the year; but you might instead consolidate it with your "Blueberries" category to make a "Berries" category that has more heft, and a longer season, and more reason to write every month.
If everything you write fits into only one category, maybe it's time to start a new blog that focuses on that topic. That's what I did when a few posts about library systems turned into my Superpatron weblog, which in time led to some travel and speaking engagements. Some of the topics that I brought up that would never have attracted any attention as part of my general weblog, but were consistently on-target for a specific audience that appreciated the attention and the thoughfulness focused on a single topic. When you set boundaries, you get the freedom to explore within those boundaries and ignore what's out of scope.
Thanks to Chris Lovie-Tyler for the discussion of boundary-setting.