Not sure of capacity off-hand, but it would be 10 or 11 most likely. Yes, I have seen shotguns like that in the world of 3 gun and IPSC shooting. Some of the unlilmited class guns have the mag tube sticking a couple of inches past the muzzle for max capacity. They must be unwieldy, nose-heavy beasts when loaded up, but the extra rounds are king when the stopwatch is running.
Search "ipsc shotgun" on youtube and you will find lots of videos showing exactly what you are talking about.
Mark