There really isn't a minumum. Its dependant on the amount of questions / information / resources related to the niche itself. And that would be dependant on how broad the targeted niche is. Also, I think that continually adding content is important. Just throwing a bunch of articles, information, and stuff isn't going to do it. It needs to be continuously updated. This way the content stays fresh, and interesting, both in the minds of visitors and search engines.
I've seen numbers around 40. I look at it this way, Have 10 when you start the site and then add some every few days. Building content is good from a Google Sandbox point of view. Also, new visitors will see you have a growing site and will return. You want enough articles so your site seems like it knows what it's talking about. Just rememebr to build that content.
20-40 pages are ok. however, you should update the page content frequently. you can use www.articlesitedeveloper.com to do this.
I dont updat the page content to frequently but i do try to ad more articles when i can. Is updating the actual pages already created as important too
You should constantly add more articles, but after adding 10, you could take a break and gradually add some more articles. If you have a big niche(sort of an oxymoron), you may want to have more articles.
I have seen "one pagers" do fine.. I have also seen sites that get an article every day do well. I think the key is to have UNIQUE content, and update it regularly. Then build links and you will see your profits go up.
Add a blog in addition to the main content. The simplest and most bot pulling way is adding a blog with auto pings. If you are not sure what to add pull in daily rss from populor feeds and blog them.