I think you'll find that 20 is the minimum and is widely accepted as such. A few sites require 30, but those are in the minority. And some sites (myself included) also have a maximum...mine being 40. After all, we don't want to give it ALL away.
Just declined a site the other night that had 2 galleries with 6 30 second video clips each. That's waaaay too much content to put on a single free site
