I usually do four cards, 2 are usually more complicted than the other 2 and they usually last 2 hours, so 1/2 for each card. I do each card one at a time, everyone works on the same card, passes supplies, etc and then when everyone is done we go on to the next one. I think that makes things go more smoothly than explaining all the cards at once and letting everyone go free for all. I've done it both ways and for me this works best.
Good luck, hope this helped a little