When we went into Gruul's lair, I had about 20,5k hp buffed, about 16,7k only with Blessing of Kings.
You probably can stack it up to 3-4 times, with a decent grp, I "only" got two stacks when soloing the instance.
Hope you'll enjoy seeing big numbers, i don't know if it is THAT effective for raids, because these buffs definitely need a lot of time to farm for and will be lost after death
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