I'm not sure what they played at Origins or what the official answer would be. Doing a quick spreadsheet of the two-player maps included in the Domination Rule Book though it would seem that you should have approximately one orb for every two zones in the Arena and require approximately one V'tar for every zone.
So, a 6 zone arena should include around 3 orbs and 6 V'tar. These figures are by no means a hard rule and do not take into account the shape of the Arena used or the special tiles used which can both have an affect on game length and confrontation level as well. They are merely the very roughest of guidelines.
Increasing the number of orbs would make the game less confrontational and V'tar easier to get. Increasing the amount of V'tar needed would increase game length and make mage death a more viable winning condition, while reducing it would have the opposite effect. The Orbs should be placed symmetrically allowing both players equal access to them.
Looking at images of the maps they had at Origins, it seems like they used 4 orbs, which is probably a good idea since it's hard to place 3 symmetrically in the 6-zone Arena layout they used. I would probably also increase the V'tar needed to 7 in this case though, to help balance out the easier access to it and make Mage Death more viable.
