That PiP track is just kind of a demo track. You can add the PiP FX on media, event, track or output level. And you can add as many PiP FX as you want.
But adding a frame over another is compositing/layering and if you want many frames to be layered over another frame at one time, this will always need one track for each frame, no matter if it is this particular PiP FX, keying, bézier masking or any other kind of compositing. It's litteraly putting one frame over the next. If you want to create a sequence of several PiP FX which only one frame over another at one time, you could simply use the PiP FX as event fx (or media fx).
As Marco added, the PIP FX can be added on the media, event, track or output.
Personally, I prefer trackmotion to create PIPs instead of the PIP FX. To handle more PIP's along a project with one PIPtrack, can be done by using keyframes for the several PIP sizes and positions.