How many people you need these days to get the achievement Stay Chill?
i believe you need 10 because it has to be a full raid
Recently did it with three, just needed all three of us to get the 7 stacks before finishing the fight.
You can do it with as few as you feel comfortable doing it with. Just make sure you kill anshal and rohash at the same time so you have plenty of time to sit on nezir's platform to get 7 stacks.