When launching a new directory how many categories do you think you should have? Too few and it's too crowded and unrelated. Too many and it looks empty. Where do you think the happy meadium might be?
When I first started List My Niche Directory, I had 10 main categories and many sub categories. As time has gone on, I now have 28 main categories and many, many more sub categories. I think you will find that starting with the basic categories and growing from there will be the natural order of things. The best answer though is that you will have as many as needed.