Charles M’s Answer
Here's a few thoughts to consider about your portfolio.
Show growth. Start with a very simple project and increase complexity on the ones that come after. Start small and increase the size with the ones that follow.
Keep notes on how you build the apps.
Measure the size somehow (lines of code, number of requirements, number of subsections, just some way, and be consistent so you can show growth in size, growth (improvement) in quality Example, defects per 100 lines of code, defects per subsection. )
Show growth in time efficiency. Track the number of hours it takes to develop each app. You should be getting more efficient (lines of code per hour) as you get more projects completed. Keep in mind, that you are learning new things, so those sections of your app where you include the things you are learning will not be as efficient as the other parts.
Experiment with different development methodologies. Learn how to manage the project from beginning to end. Show your project management documentation. Compare the results of the different ways to manage the project.
Perhaps create an app that summarizes the data about your other apps and a graph that shows them in comparison to each other for the different attributes you tracked.
These ideas are probably a little overwhelming. It is based on my impressions of how Boy Scouts document their Eagle Scout projects. And then extrapolated to show growth with each subsequent app you develop.
Remember, a simple app that is completed and works is much preferred over an outstanding app that isn't quite done yet, or that has a lot of bugs. Only bite off as much as you can chew and then do it again with a bigger bite next time.
Two last thoughts.
If you use the Agile Development Methodology, your growth can be shown with different versions of the same app. The first iteration, may just open and close. The second iteration, may just have two functions. Third iteration may have 5 functions. and so forth, Keep them as separate apps so you can show growth.
Keeping all this extra data about your development processes and growth is great, but also having a well rehearsed presentation about it makes it much, much better when talking to those interviewing you for a job.