Take EM machines into consideration, I've seen my '72 King Kool with stock lighting go from 1.8 amps at rest to 7 amps while in play. With that taken into consideration 2 EM games could be played at the same time on a 15 amp circuit with little left available. Most homes built today have 15 amp receptacles installed with 20 amp passthrough on the back of the plug so even though you have a 20 amp breaker installed that does not mean the receptacle is rated for the full curent, the breaker rating is the total sum current of all plug receptacles on a circuit.

Circuit breakers can handle more than the listed rating to cover inrush currents for starting and equipment powerup for a short duration but will trip when the current goes beyond the listed rating for a longer duration of time. Running a circuit at its maximum circuit breaker rating using one receptacle for the entire load will stress the connections at the plugs and breaker resulting in metal fatigue eventually burning connections causing the circuit to fail sometimes catastrophicly.

If using plug strips for devices such as pinball machines make sure they have the same load rating as the circuit it is connected to or do not exceed the watt rating listed on the plug strip, most plug strips (The cheap ones) use smaller lead wire such as #14 or #16 and will typically fail at the face of the receptacle due to the heat generated from the load flowing through the smaller wire. In the end treat each wall receptacle as being able to provide 15 amps maximum while all plugs on the circuit should not exceed the wire and or breaker current rating (whichever is less).