I'm going to take a risk, and say only 4. He'll get bored eventually. Plus with next years Renault engine being (allegedly) massively down on power in comparison to merc and Ferrari, it'll be harder for him to win.
6, he's only 26 so he's got another 10 years at least but the Red Bull dominance cannot go on forever. I reckon at some point he will move to Ferrari or Mercedes.
He'll already hit that target by the end of this season. Hard to say if Alonso and Hamilton will notch up any more.
5, he'll win this year, then his car won't be so dominant and maybe win one more before his career ends.
If he has a few years of not winning the WDC, he'll go back to spoilt brat mode and over-drive. Still reckon he'll get at least another 2 after this years has been won.