If we were talking about holograms or something, then I could see adding the extra dimension, but the 3DS isn't 3D in the same way the real world is, it just appears that way through optical illusion. If you wave your hand above the screen in the same space the image appears to be popping out into, you're not going to actually hit anything. It's the illusion of depth, not actual depth.
If one were to say that, one would be wrong, because there is actual depth in the real world, which there is not in 3D movies and such.
And there is such thing as monocular depth perception. It's less accurate/more difficult to see actual distance (That is, you see that there is distance between two objects, just not how much/how far the distance between the objects is), but you don't need two eyes to see the real world in 3D.
Basically, the 2 camera angles are flashed into each of your eyes so you see 2 different angles for an object. For a better understanding, close one eye then alternate, you should notice different angles. That's how your eyes view 3D. It's still 2D images but, since one is flashed to each eye, you see different angles much like how you would in real life, creating the 3D effect.
"so theres no number for the 3d pixels? this confuses me."
3D pixels are called voxels. I think the only way to measure voxels in a game would require measuring how far before the view culls, the resolution of the screen, and the angle of the FOV of the player, but it's not exact. You may even have to double the total(400x240 would be what we would consider the screen resolution if we are going to assume two POVs equal two sets of voxels).
One thing we can count a lot easier is polygons.
"flashed" makes it sound like the images are alternating. The 3DS renders and displays each frame at once to both eyes. Technically, everything on a screen "flashes" frames to create the illusion to movement, but it's not neccesary to note unless comparing to a media that doesn't "flash".