Personally, I've been stocking my portfolio with 2 subjects (portraits and nature), using my best medium (charcoal). In my case, portraits sell much more frequently, but the landscapes and close-up nature pieces garner the most attention and best reactions. So I alternate between the two. But I've also spoken to artists who load up their portfolio with many different mediums and styles in order to show off their range, and it works for them. I think it heavily depends on whether you're selling individual pieces to clients, or if you're looking for steady work in a specific area. For instance, if I was going to an interview for a position where I had to design office furniture, I'd make sure my portfolio was loaded with that kind of stuff. If I'm talking to a business owner about doing a commission for office art, I want them to see a variety of subjects, so they have more to choose from.