If there is only one outlet on a 20 amp breaker, the outlet must be rated for 20 amps per NEC. We are allowed to use 15 amp outlets on a 20 amp branch circuit because the notion that your never only going to use one outlet.

Yes. You can tell by the horizontal/ vertical prong on the hot side.

article 210.21 (B) 1, 2, and 3 of NEC.
Yes duplex outlet (two plugs) is already considered "multiple outlets" so it's permitted on a 20amp breaker. Part of being a UL listed outlet includes that a 15 amp duplex be capable of feeding through 20 amps of current.

Other things to consider, hopefully a 20 amp circuit uses #12 gauge wire and the outlets are not "backstabbed".

No worries. Had to find the reference after you said wrong to make sure I wasn't giving false info.

If people are making game rooms you can have your electricians wire the upper outlet of a duplex to one 20 amp circuit and the lower to another. That way you have two circuits at every outlet to even out the load.

That being said I have a police force, high speed, and baywatch on 1 15 amp circuit and all is fine. No led in the sys 11's yet.

Another thing too is most libraries have kill-a-watt Meters to checkout to find peak power consumption of what's plugged into it. That exactly will tell you which games can on on a particular circuit