A gun with a six-man crew would, I think, have been a fairly small one - maybe a 12- or 18-pounder. Larger guns (24- or 32-pounders) had bigger crews of up to 12 men - maybe numbers 2 & 6 were those who led the heaving of the gun back into its firing position (it would have needed a lot more than just the two of them, unless the ship was heeling to their side).