I have nearly as many shotguns as handguns, one may ask why, but IMO you really need 1-5 different gauges depending on what you do. I have a 410 for small game and 20ga for grouse, and dove, although many use a 28ga for dove, and then a 12ga for waterfowl, and upland hunting. I would also probably add a 10ga if I hunted Canadian Geese frequently. You can do it with one shotgun but to be honest you should really have a 20ga and probably a 12ga. 20ga for smaller animals and 12ga for larger game. Also here we deer hunt with shotguns so I have another shotgun only use for deer hunting currently I use a 12ga for deer, but I may be going back to 20ga just because the technology in 20ga has gotten very good in the last few years, and the recoil is far less in 20ga, and basically there is really no advantage to the 12ga for deer.
So to some this up
1-12ga pump for self defense
1-12ga semi-auto for waterfowl, also with different barrel used for 3gun although at some point I will be going away from a gas system, to an inertia system for 3 gun so that will be another shotgun, I also use this shotgun for skeet
1-20ga pump for grouse and dove, but many use a 28ga so that would be another if you so chose, I don't dove hunt a lot so no need IMO
1-12ga O/U for upland, and trap
1-12ga semi auto for deer
1-410 for break action single shot for small game
So for me I need a minium of six shotguns, and I own many more than six. If one were to have a inertia gun I could probably limit it down the number of shotguns, using 1-12ga for waterfowl, home defense, 3gun, deer, and skeet, but the expense for barrels would be prohibitive. This is all only my opinion