Sorry if it has been mentioned, I just skimmed, but it would also dependent on scene/context. Someone like Steven Erikson who is weaving this gargantuan epic history has so many characters it truly is astonishing. In certain sequences he jumps from head to head to head to head to head to head to head you get the point, but always when there is a good reason for it (like seeing a cataclysmic event or whatever through a number of differing POVs so the reader gets a multifaceted view of whatever is going on).
Martin gives each POV its own chapter so it is relatively easy to keep track of stuff (much easier than Erikson at any rate, who often can start new sequences without revealing who we are supposed to "look through").
Personally I think the more, the merrier