How many iteration should be used for collision detection

When handling collision contact, collision constraint should be iteratively solved. However, I have a question that what kind of collision constraint should be iteratively solved ? Obviously, there are some kinds of collision constraint do not need be iteratively solved. Can someone explain to me what kind of role iteration play in collision constraint solving?

Most physic engines solve constraints iteratively because solving all constraints together is not practical in real time. They trying to solve one constraint at time and use iteration to cover a global solution for the system.you can google for Sequential Impulse Solver