Based on our experience helping hundreds of applicants, the ideal number of projects to include in your PMP application is three. Listing fewer than 3 projects provides less opportunity to document the experiences that the application reviewer will be looking for. Listing more than 5 projects increases the risk that the reviewer will classify the projects as "ongoing operations" rather than formal professional projects. You will need to show a total of 36 months of project experience (or 60 months if you only have a high-school diploma).
Once you meet these minimum requirements, you will not be allowed to add additional projects to your application.