Whats the driving force behind simple diffusion? Is it just the count of molecules?
All particles are constantly moving randomly due to their kinetic energy, and are thus constantly colliding into each other and the walls of their container (if any).
Particles in gases move rapidly and randomly and are spaced far apart, particles in liquids slide slowly over each other, and particles in solid vibrate around a fixed position.
Hence, diffusion can be defined as the overall movement of particles, from an area of higher concentration to an area of lower concentration, down a concentration gradient, due to their random motion.
Illustration of particles in a gas:

Source: wikipedia.org/wiki/Kinetic_theory_of_gases#/media/File:Translational_motion.gif
This explanation relies on Kinetic theory and Brownian motion.