Firstly, It depends upon which area you wish to specialise in. As I specialise in children’s literature, I can only advise on what to put into a portfolio for this specialism. There are of course cross overs with every type of specialism. Some illustrators specialise within a specialism, one may specialise in people another in animals , although a good artist should be able to be proficient in both. In the beginning you should aim for between 15–20 pieces of your best work. and hopefully you will have already found your style, because a portfolio should be in a consistent style. This is your trade mark, it is your personal identity. The subject matter is important , although this  can vary from illustrator to illustrator, however, In general, a good way to think of it is as it is in life, ie People: You should try and include young children, adults, and the elderly. in different poses and with different emotions. different ethnicities. Animals: young and old, different emotions and anthropomorphism. flying, etc. environment: rural, urban, night and day, weather: water, rain, Fantasy: unusual situations (imagination) Children like the unusual, they delight in the profound and the crazy. creatures, monsters, mythical creatures, try and include dynamism, movement, speed, odd angles, unusual positions. (difficulty in drawing) there should be Character designs. some monochrome work. spot illustrations, some half page, full page, double page spread, book cover. You can include a piece of work with a few of these elements in one image. Every illustrator is in a constant flux with their portfolio , and it evolves over time, as your work , style and working methods/techniques mature you will find your work will become more complex and dynamic, and you will drop images that once you thought where your best work. Another technique to show a lot of different scenarios and characters is to have a seperate section on your website for drawings. As I have done myself. This also shows your aptitude for drawing which is important when sending plans/visuals to clients, who like to see them in your site. Take a look at my website, I have around 30 images at this moment in time, I have tried to show something of everything, in my own style, that is all you need to do yourself. potential commissioners want to see consistency, (own style) and a breadth of life scenarios if you achieve this, you will have a good portfolio.