Supply value and town defense

[manesh 1]

Hi,

playing some warfare with a friend and I wondered how the supply value of a town correspond with the defense of a town. Because when I build defense weapons around the town they most of the time will not be manned and thus be useless.

So what determines how many available men a town has to man those defense guns? Would be nice to have absolute numbers; like x value = y men for defense guns.

Thanks