Sometimes focusing on one niche and one forum (or at least focusing most of your energy on one) is a better idea. In that case, you can really promote and build it. But that doesn't mean you cannot work on others later.
If you mean you have several forums to handle, I don't think it's a good idea for you cannot focus on all of them. Maybe you create one forum first and give it your all. When it is doing good and doesn't need much of your attention then that's the time you can create another forum to focus on.