You can't fit 14 million bodies in one place at one time.
Broadway Avenue in Yonkers is about 105 feet wide, so shoulder to shoulder you could fit 70 bodies across it. With only a 50% hit rate, a single M134 minigun could kill 21 ranks of zombies a minute, or a horde of 1500 about 45 feet deep.
If a zombie can move at human walking speed, they can advance at 270 feet per minute, so a battery of 18 miniguns, allowing for 6 firing concurrently, could kill the horde faster than they can advance. Probably fewer guns would be necessary as the dead bodies would slow the advance as they stacked up.
And that's just an example of a single type of gun. The Mk 19 grenade launcher's standard ammo has a kill radius of 5 meters. Call that an effective disablement (outright dead or unable to move even if alive) radius for zombies of 2 meters, and a single round can stop 50 zombies in a fairly dense pack. At 40 rpm sustained, that's nearly 2500 kills a minute.
This is easily firepower an infantry battalion is capable of unleashing on an unarmed, dumb enemy willing to merely advance into fire. It doesn't include the kinds of kills that could be added with artillery, high explosive bombs, etc.
A Mk 82 500 pound bomb has a kill radius of 100 feet and a single B-52 could carry 52 of them. An overlapping string could kill a horde a mile long, packed densely nearly 800,000 zombies. Carpet bombing by B52s squadrons could kill city-sized zombie hordes in minutes.