On what basics atmospheric pressure of a planet depends.?

M and radius determine the force of gravitational field at the surface, which determines how much atmosphere a planet can hold on to. Distance from the primary star, combined with the initial composition of the atmosphere as well as the early geological activity of the planet determine whether a planet will end up with a runaway greenhouse (like Venus) or not (like Earth...so far) and whether lighter elements will persist in the atmophere. Specific composition of the planetary disc and the presence of volatiles also determine atmospheric composition.