Webdev summed it up well, there is no set number of pages you need. Some sites have 30, others have 300+. I was very fortunate to find a niche that allowed me to have success so quickly with only 30+ pages but I do plan to build more... I've just gotten too comfortable with its success and have been working on other things.
I think the fact that you've noticed there aren't a lot of "how to" portrait sites is a great opportunity to capitalize on that niche. When I create new sites,I try to find other themes and content that other sites don't offer so my site stands out and you're off to a good start.