There's some truth to the old saying: "Beware the hunter with one gun—he knows how to use it." On the other hand, many hunters would argue that shooting only one shotgun takes some of the versatility and a lot of the fun out of the whole enterprise.
Here are some thoughts and recommendations on both sides of the debate.
One Is Plenty
Using just one shotgun lets you learn that gun's balance, fit, dynamics, controls, and trigger pull intimately. There's no fumbling to find the safety or to quickly reload. You get the same fit and feel every time. It's a bit of a cliché to say that the gun becomes an extension of yourself, but it can seem to point and swing without conscious thought.
The one-gun hunter needs a gun that can do everything. It has to shoot light loads in September and heavy loads later in the season, all while shrugging off the worst conditions. Consider the following when choosing a do-it-all shotgun:
Gauge. You can't argue with the 12-gauge. A 3 1/2-inch 12-gauge lets you shoot everything from 1-ounce loads at clays to 1 9/16-ounce loads of BBBs at geese. A 3-inch 12-gauge handles all but the longest shots just as well as a 3 1/2-inch gun. The 3-inch gun also might cycle better with very light loads, and it will cost less.
Despite its current popularity, the 20-gauge is a niche gun, although the niche it fills is wide. If you only shoot decoying birds, a 20 could be your only gun, but the 12's versatility makes it the better choice for most shooters. The 10-gauge fills a very narrow niche. It's not a gun for jump-shooting, long hikes, or small ducks, and you can't readily find target loads for it.
Weight and handling
Many 12s on the market are almost as slim and light as some 20s used to be. A super-light gun is fun to handle, but it's challenging to swing well on longer shots. Most shooters do best with a gun in the 7- to 7 1/2-pound range, and that usually means a 12.
The winners in the recoil contest are 10s and 20s. A 10-gauge reduces kick through its sheer weight, while a 20 can recoil less due to lighter loads. Lightweight 12-gauges aren't painful to shoot with standard 3-inch loads, but they kick horribly with 3 1/2-inch loads. Recoil-reducing innovations like Beretta's Kick-Off system, recoil pads, and soft comb pads make shooting heavy loads in light guns more bearable, and you can always choose a gas gun to reduce felt recoil even more.
Cerakote, Benelli Surface Treatment (BE.S.T.), and Beretta's Aqua Technology protect guns from the elements, including salt.
Any number of guns check all the above boxes. Pick one and practice with it. Shoot it during dove season and you'll be dialed in for bigger birds when the time comes.
One Is Never Enough
It's easy to make a case for more than one gun. Even the most reliable guns have problems sometimes, so it's a good idea to have a backup. And if you like hunting with valuable old guns or heirlooms, you'll want to shoot something else on those days when weather or other factors might harm that special shotgun.
A 20-gauge is fine for shots within 30 yards at ducks and geese alike, and it's great for teal season. If you want a 20 just for those hunts, why not? Find one with some heft that you can swing well. You can also make a strong argument for adding a big gun to your quiver if you pass-shoot Canada geese or shoot backup in the goose pit. A heavy 3 1/2-inch 12- or 10-gauge kicks much less than a light gun. The 10 in particular patterns big shot better than most 12s.
The best reason to own more than one gun is simple: We do this for fun. Maybe it pleases you to stand in front of the gun cabinet and ask yourself, "Which one should I shoot in the morning?" Having more than one gun may not put more birds in the bag, but it might help you enjoy hunting even more.