If you have a body of work that is large enough to include 50 images, then you probably have enough to put together something reasonably focused. If for nothing other than the purposes of an exercise, start with a really broad category, like color or black-and-white. Throw down EVERYTHING you've printed under that heading, then start organizing. I'm sure you can come up with some micro-groups, say 3-5 images per topic (landscapes, abstracts, automobiles, kids, whatever). When you start thinking about groupings, you'll see a tremendous fluidity between them, but also an amazing continuity as well. That's probably how you can best pull together a portfolio. I think you'll find inspiration for your future shooting from this exercise, as it will give you a bit of direction. You'll be able to look at your work and say, "Hey, I really like this image/group, I want to explore that more and expand on that idea". So, even if your existing work doesn't gel and give you a 20 image portfolio, the process of figuring that out will help you get there.
Also, if you are looking for a portfolio as a gift/representation to your parents of your work, who says you have to have 20 images? Ten to fifteen would make a more than adequate grouping.