Intuitively I agree that more than 3 would really suck, however I haven't played the first one yet so who knows how they're going to handle it. All I will say for certain is that each installment has to stand on its own two feet and be a complete, satisfying narrative in itself. If they can do that, maybe I could cope with more than 3 for 20% of my overall life expectancy but I'd be surprised.