If you want to steam artichokes evenly, you should start with a pot large enough to house all the artichokes side by side, without having to stack them.  This will ensure an equal amount of moisture gets to each artichoke.

Unfortunately, this creates another problem: with plenty of room, the artichokes may tip over causing half to be submerged and again, causing them to cook unevenly.  There is a quick fix you can employ to prevent this, too.

Slice an onion between one and two inches thick.  Remove the outer rings and place them in the bottom of the water filled pot.  Place the artichokes in the center of the sliced onion rings.  Not only will this keep the artichoke from tilting left to right, but it will buoy the stem of the artichoke above the bottom of the pot, ensuring it does not overcook!

Happy steaming!