Yes, on helos, the 2x or 4x stands for the number or rockets fired in one salvo. It is just a more convenient way to acnowledge a weapon fired in bursts. Helo rocket pods have a very high ROF, salvo lengths tend to be programmed so you cannot fire one at a time. And basic physics have it that in a hovering heo, firing rockets in pairs from two launchers prevent both helo and shot from going astray.
I guess it is also meant to reflect the number of actual rocket packs on your helo.
That doesn't influence the ammo loadout. Loadout is "number of shots" whatever the shot is. So actually, if you have, say, an Apache with two Hydra pods with 19 rockets each, it will have 19 shots of the weapon "2x 2.75in. FFAR". Could as well have 38 shots of 1 rocket each, but that would be much easier for him, don't you find?
Same for ALL automatic weapons, Kevin...
20 shots of a .50 MG doesn't mean it carries only 20 rounds, but that the average burst length is so, that there is enough ammo stored for 20 bursts.
I don't know how the helo gunners use their MGs, but if a burst is 20 or 25 rounds, so you can aim at anything at a decent range with the helo vibrating and so on, that already gives you 400 to 500 rounds, which seems quite good for a door-mounted MG on a transport helo where every pound counts.
This rule works for anything that fires bursts, from assault rifles to gatlings. The longer the burst, the bigger the damage, but you cannot decide where you put every single round from an Avenger gun, can you? Besides anything else, you would end up with 500 shots to place in one game turn!