But seriously folks...
Generally they recomend #1 Buck.
Firearms Yactical Institute:
20 Gauge Shotshell Ammunition Recommendations
We're unaware of any ammunition company who offers a 20 gauge shotshell that is loaded with #1 buckshot. The largest shot size commercially available that we know of is number 2 buck.
From a strict wound ballistics standpoint, we feel the Federal Classic 3-inch 20 gauge Magnum number 2 buckshot cartridge is the best choice. It contains 18 pellets of number 2 buckshot in a plastic shotcup with granulated plastic shot buffer.
However, the Federal Classic load might produce too much recoil for some people. Given this consideration, Remington's Premier Buckshot 2 ¾-inch 20 gauge number 3 buckshot cartridge is the next best choice. This load contains 20 pieces of nickel-plated, hardened lead shot that is buffered to reduce pellet deformation from post ignition acceleration and terminal impact. The Remington buckshot load will probably produce the tightest shot patterns in 20 gauge shotguns.
Third place is Winchester's 3-inch 20 gauge Magnum number 3 buckshot cartridge, which contains 24 pieces of buffered, copper-plated, hardened lead shot.
"Number 1 buck is the smallest diameter shot that reliably and consistently penetrates more than 12 inches of standard ordnance gelatin when fired at typical shotgun engagement distances. A standard 2 ¾-inch 12 gauge shotshell contains 16 pellets of #1 buck. The total combined cross sectional area of the 16 pellets is 1.13 square inches. Compared to the total combined cross sectional area of the nine pellets in a standard #00 (double-aught) buck shotshell (0.77 square inches), the # 1 buck shotshell has the capacity to produce over 30 percent more potentially effective wound trauma. In all shotshell loads, number 1 buckshot produces more potentially effective wound trauma than either #00 or #000 buck. In addition, number 1 buck is less likely to over-penetrate and exit an attacker's body."