Add forums as the need arises. If a lot of your members suddenly get the urge to discus 'music related topic' then there may be a demand for a 'music forum'. You can always move topics in there from others.
By the same means, don't be afraid to shut down a dead forum. If nobody has posted there in a month, then you should consider moving the old topics into another forum and closing it.
The 'goal' is to have a fair amount of activity in each forum.
