Question

Why a solid can have any number of free surfaces and a liquid has only 1 upper free surface and gases have none?

Answer 1

Solids have a definite shape and volume, liquids have a definite volume but not shape while gases have neither a definite volume or shape.
Gases take the shape of the container they are put in and occupy all the space available to them and hence they have no free surface, while liquids are bounded by the container they are placed in and have only one free surface, the upper surface.