that depends on a lot of things.

1. how often will you be updating?
2. assuming you will offer video, how long will the average video be? if they're really short due to the niche, you'll want more of 'em.
3. will you be offering high rez pics?
4. will you be offering other content beside pics and videos?
5. how micro is the niche and how many other sites are there in that niche?