How to let grow PingPong balls inside an invisible box? Inspiration / Reference
So I came by this interesting example of ping pong balls from Alvaro Navarro, where the amount of spheres are growing inside an invisible box.
So I think he is using some kind of particles to generate new PingPong balls at a point down the bottom of the invisible box, where the new generated particle balls will be pushing the already generated balls to the top using some kind of collision.
Tried
To make a simplified version for the first step, I did the following:
So I created a box. Deleted the top face and add Rigid Body Passive (so the box doesn't fall down). For simpelicity, I added one sphere with also the Rigid Body, but now type Active. Unfortunately it seems like it will set this type for all objectes with Rigid Body.
Something that is made in C4d, but don't know if this is possible in Blender, because there is no collision for each particle I've read? https://www.youtube.com/watch?v=HoxBLXKZ7gw
You can achieve a similar effect with builtin physics engine.

Just add a bunch of spheres to your scene, set physics type to Rigid Body, Active and collision shape to Sphere. Set it up in advance before duplicating, or copy it afterwards.
Place them inside an invisible container with a simple box-like shape, like the default cube.
Now you need to push them against a wall. Do so by either animating the container shape, or optionally using a second animated object to push them around.

The illustrated example uses an animated squashing box. In retrospective you could probably animate its scale instead, avoiding changing shapes and expensive mesh morphing physics, but this example uses shape keys.
Set the cube mesh up with Rigid Body type physics, with both Dynamic and Animated active, and collisions set to Mesh type, Source as Deform and Deforming option active.

Add one additional shape key besides Base, for that shape key move the back face forward enough distance to squash the desired amount of spheres against the front face. Keyframe the factor for that shapekey from $0$ to $1$ for the desired duration of the animation.