I was told that engines were more stable in multiples of 6's from some weird physics going on in the engine. I guess that guy could have been completely full of shit.
That guy was right that there are 6-cylinder configurations with perfect mechanical balance (and not for other cylinder numbers), but the V6 isn't one of them so that's not the reason here.
Its mostly a trigonometry thing, three sine waves 120 degrees out of phase balance out to constant torque. Also see electrical 3-phase power.
Note that internal combustion cylinder pressure is by no means a perfect sine wave LOL but its a close enough approximation to run smoother.
Its partially a physics thing, in that ICE are usually 4-cycle so they fire every other rotation so if you want three firing pistons per rotation separated 120 degrees you'll need six cylinders total.
