The key to success is diversity
I have only been with DT for a sort time and I only have 47 images online but I fell like I have had very good success for a beginner with 12 sales so far.
Most of my sales came when I had a lot less images on line, about 35. With so few images I think that my diversity of images is what has given me the sales I have so far.
I see some contributors like to specialize in certain types of images but that will limit your ability to sell photos to a wide variety of designers as designers are not always looking for the same types of pictures. By having a large variety of good images I would think that your portfolio would get more exposure, i.e. show up in different searches and lead to higher sales.
Photo credits: Mvogel.