Generally, if you are trapshooting, you load only one round into the gun, or however many you are going to shoot right now during your turn. You don't have the shotgun loaded when shooting trap unless it is your turn to shoot.
I am not sure if the law is state or federal, but everywhere I have ever hunted, the law was that when you were hunting with a shotgun, it had to be plugged so that it could hold no more than three rounds of ammo. This was when hunting any game animal with a shotgun: for example I recently hunted whitetail deer in the state of Ohio with a shotgun and the rule said that the gun can only hold three rounds.
Other than hunting, or the rules for a particular shooting game, there is no federal law on how many rounds you have have in a shotgun. I have a shotgun with an aftermarket magazine extension that holds 10 rounds. I shot a local plate match with my defenisve shotgun that holds only four rounds. There were 7 plates, so I was at a severe disadvantage since I had to do a reload. So, when I got home, I ordered a 10 shot extension to be used with a my 28" hunting barrel. I put it all together and looked at it for a week or two and then went back to my original configuration without me ever having tried to fire it with the long magazine extension.
