I've seen people start up new forums with tonnes of sub forums - every conceivable category - all empty.
While it's true that they were logically defined, they had no purpose and it would be a long time before they would have.
So start small - have a few categories and build from there.
Over time, your main subject area forum will build up much faster and bigger than the rest - it's the primary focus of your community.
So have a look at the subjects of the threads that are posted there - if a fair percentage of recent posts are all on one sub-category, then create a new forum, move the topics and voila! The perceived size of your community just increased.
Now, because this new sub topic is actively discussed, the chances are it will also be one of your better search terms or at the very least it will be a popular search term (even if you dont rank well).
There's the next step for you to do - optimise your new forum for this new keyword.
6 to 12 months down the line, you should still have the general forum you start with, but now it will only be used for unique stuff that doesnt fit anywhere else.
The majority of your members will be using the forums that are more specific (eg Webmaster General vs Website Technology Issues).
On top of this, you are targetting the broad term for your industry with your general forum (generic term / highly competitive) as well as multiple sub-category terms with your new forums (semi-generic terms / moderately competitive) and niche terms with the threads your users post (specific terms / varying competition).
Furthermore, by this point (12 months down the line) you have been a good SEO and built up your traffic and inbound links, etc etc etc so now your generic forum should be ranking pretty well and your more specific terms should be number 1! ;)
A little bit of planning in the beginning can save you time re-arranging forums later on and if you do it right, it can be an excellent source of traffic.