How to measure 'grip' of a tire:
Well tire 'grip' is useful in two directions: longitudinally & laterally. The first being forward/backward, and the second being left/right. The combination of these two directions is what allows the car to travel in curves.
Tires have a coefficient of friction... that changes depending on the forces in the Z direction (up/down).
Forces in the Z direction can be related to weight of the car, aerodynamics forces, unsprung mass of the car, etc. All of the different forces affect the tires differently when coming from different sources. This you can learn more about in further study of vehicle dynamics.
In a corner, your limiting factor is lateral acceleration. Let the 'grip' forces be define in Y direction (left/right).
ForceY = Coefficient of Friction * ForceZ
Note that more ForceZ will increase ForceY with a constant Coef of Friction. However, take note that earlier I said that the Coef can change depending on the properties of the tire and how they respond to changes in load.
This is why Aerodynamics play such an important role in going fast. Maximum Mechanical 'grip' in lateral acceleration is around 1.7 g's. Maximum Aerodynamics 'grip' can exceed 4-5 g's. Tires are a big deal.
So, in conclusion:
Tires are complicated, and dynamic. They don't have a 'max grip'. Well they do... but it's only limited by factors that are wayyyyyyyy out of this scope.
Good luck, you have a lot of reading to do.