It’s capped at six this week not because that’s the total amount of Vessels you can acquire, but because Wrathion simply won’t give you the quest for rank 7. Next week, he’ll start giving out the quest for rank 7, the week after that, he’ll give out rank 8, etc. This is a universal timegate, not a per-character one—if you start late, you can get as many upgrades as you have Vessels until you catch up.
Source: Ashjra'kamas, Shroud of Resolve in the Weeks to Come