What goes in a portfolio?

When building a portfolio, how much should be in it per project? I’ve heard to have 1 or 2 pages per project with just the essential information and i’ve heard several pages, as many as it takes to display the details of the project. so, i’m a little confused. should research be included? some, little or no text? rough sketches, refined sketches? What are some qualities of a strong portfolio besides the actual project and the graphic display?