The maximum cell density for a 1L starter is approximately 200B cells. Twelve gallons of wort is approximately 45.5 liters with maximum cell density of 45.5 * 200B = 9.1 trillion cells. You want your step rate to be no more than 20 to 1 and you need to aerate your wort very well because a) it is more difficult to dissolve O2 in high gravity wort and high gravity wort places high osmotic pressure on the yeast cells. A step rate of 20 to 1, yields log(20) / log(2) = 4.32 replication periods to reach maximum cell density for 45.5 liters when pitching a 2.275 liter starter . If we push the step rate to 22.75 to 1, we will have log(22.75) / log(2) = 4.5 replication periods for the batch of wort to reach maximum cell density, which is in the noise. If you aerate your wort very well and pitch at high krausen, two 1L SNS starters will get the job done. Whatever you do, do not wait until your starters have fermented out to pitch because that will increase the dissolved O2 requirement for the yeast culture. You want to pitch propagated yeast cells while their cell walls are still healthy and they still have some ergosterol and unsatturated fatty acid reserves, which means pitching at high krausen. Waiting until a starter has fermented out before pitching is a bad practice because the yeast cell have undergone morphological changes that need to be reversed before they can start to replicate. We especially need to pitch yeast cells in prime condition into high gravity wort because difference between the solute level inside of the cells and the solute level in the wort will cause water to be drawn out of the cells (a.k.a. osmotic pressure). When this happens, the plasma membranes of the cells lose turgor pressure, which causes the cells to wrinkle up like raisins. If the cell walls are not strong and the cell membranes are not pliable, the cells will literally implode. Pitching a higher volume of yeast cells will afford a certain level of cell death, but it is not a replacement for adequate aeration and pitching healthy cells. Plus, the difference between a 2L starter and 4L starter is one replication period because the yeast biomass grows exponentially, not linearly or multiplicatively.