Every ship allows you to mount only one armor at once. You can carry multiple ones, but you can't mount them all, and will lose/drop them if you get killed.
You cannot mount two armours. You can just carry any much you like, unmounted tho. And that too, depends on the armour capacity and your cargo capacity.
So, just one armour and the rest are unmounted/will be lost once you are dead.
You can buy them at quite a few locations, mainly shipyards yes. I suggest downloading Freelancer Companion to get a full list of locations for the armour.