How is the strength of London dispersion forces related to the number of electrons?
London dispersion force (LDF) depends on the surface area of interacted particles. Moreover, more electrons results in larger atoms size and therefore, stronger LDF.
This is also derived from the fact that higher number of electrons will increase the chance of forming what is called induced dipole (also called polarizability) which will make the interactions stronger.
Source: Zumdahl Textbook .