You can mix different projects in the same portfolio, but it would be best to limit the number of projects represented. In a 20-image portfolio, don't have more than 3 projects represented. Any more than that and it looks like you're not able to concentrate on a theme and develop an idea. This is just one opinion. Bear in mind that the guidance from graduate school applications is that (generally speaking) they expect a 20-image portfolio on a SINGLE theme. Galleries may or may not have this same expectation - depends on the gallery. Submitting a portfolio to a gallery like Fraenkel or Bonni Benrubi or Modernbook, they will be the most critical of the work submitted, whereas a small gallery outside of a major metropolitan area would be more likely to be flexible, and your neighborhood coffee shop will probably only care that they like the pictures. Your best bet is to prepare for the most rigorous standard, and then you'll stand out anywhere that doesn't have as stringent a set of requirements.