While there might be a limit, I have never encountered one. I have used up to about 150 PiP Images, and maybe 50 Videos, with no issues.
Now, one could possibly run into issues with the computer's resources, but with a stout video-editing computer, there should be no issues with the 8 - 12 PiP's.
As I recall, there are 99 video tracks in Premiere Elements 12 Expert workspace. So your 8 to 12 picture in picture effects do not appear to be a problem. Computer resources and pixel dimensions might complicate matters the larger the project.
What is your vision for this project....a background video on Video Track 1 and periodic picture in picture (on upper video tracks) presenting in the video as the Timeline duration proceeds or something else?