The NEC doesn't specify. Generally they are installed vertically with the ground on bottom.
In the world that I work in the ground is always installed on top. This is so if a gap exists between an outlet and the plug, anything falling into the gap hits the ground first and not across the hot and neutral wires simultaneously. Expanding on this premise an outlet installed horizontally with the neutral on top is the most safe installation. But I have never seen this done.
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No. In a 20 amp circuit all wire has to be 12 AWG or larger.
Receptacles are not wired in series. Receptacles are actually wired in parallel, what this means is that all phase wires (black) in a receptacle's box should terminate to the brass screws on each device, and all neutral (white) wires in the box should terminate on the silver screws on each device.
A commercial receptacle that is used to prevent arching.
In the United States and according to the NEC, in commercial and industrial installations, you are limited to 10 receptacles on a 15 amp circuit. The size of the wire is not a determining factor. There is no limit to the number of receptacles on a circuit in a home and there may be local codes where you live that have stricter requirements.
Electrical wall outlet having two plug receptacles.