What is the best way to remove a project? Does it create any problems if completed projects begin to accumulate? Should I remove completed projects when I'm done with them? Thank you.

Keep as many as you like. They are just folders with files. Delete at your pleasure.

Well, therein might lie a problem for the OP. There's a possibility that he may not have all his projects each nicely assigned to a folder. It's possible that all the WAV files are stored in a generic default Reaper media folder, along with the project files.

Umm that would be bad! OK take each project and save as and click "copy media items" and save to a separate folder for each project, then remove the original folder.