How should my strategy change for six players?
Call easy, Play aggressively

I've only played it up to 5 players but in my experience the game works best if you don't go overboard with you calls. I prefer to call one or even two tricks less than I tought if someone else calls big numbers. But then I try to persue those tricks aggressively.

Because even if the calls add up nicely people generally don't manage to follow through. So waiting towards the end to get you tricks is risky as the chances that someone messes up increases with the number of players. Fighting for you tricks to soon is risky too because that's an easy way to waste good cards to wizards.