In my opinion, you should start with less categories and add more when the number of listings is high enough to warrant adding more. You can then add categories as needed, wherever it makes sense in your directory.
Empty categories are empty pages, if i could go back to 2007 and start my directories again then i wouldn't add categories without having something to put in them first. Nobody makes blank blog posts yet in directories theres empty categories everywhere.
I created a lot of categories in my directory. My goal is get more indexed pages, more inbound links, different keywords and reach content. Bu I am not sure to reach my goal because its not a old directory. I will wait and see. Note: I see that now, www.romow.com/blog/3-column-wordpress-theme-mistyblue/ page has virus (Google: "This site can harm your computer.") but others not. Interesting!
How long is a piece of string? The number of categories is going to be directly proportionate to the theme or nature of the directory and what is listed. It's impossible to say.
if there are too many categories, each category has only 10-20 links and does not consist sub-pgaes under the categories so much. I think this is an advantage for the site owners. take high PR and indexed so hard for the sub-pages.