Good question- this is by far the most stressful and important part of applying to an art program.
As an art professor, I'm always looking to see if a student has a strong foundation while also being open to experimentation. If an application asks for 20 examples of work. It would be good to include 10 images of work you did for a class that shows your technical skills (Intro to Drawing, Basic Design, Photo I, ETC.). The other 10 images can also be something done in class but showcase more creative freedom. Maybe it was a self driven project where you could decide on technique, medium and concept.
What you include should be largely influenced by the specific program. Take a look at the type of work their students produce. If it's a school that largely favors abstract work, it might be unlikely they'll choose someone who focuses on hyper realistic portraits.
Feel free to reach out and schedule a session with me to go over your portfolio and goals!