The key factor that determines the amount of inertia an object has is its mass. Inertia is the property of an object that resists changes in its state of motion, whether that be at rest or in motion. The greater the mass of an object, the greater its inertia.
Mass can be defined as the amount of matter present in an object. It quantifies the object's resistance to changes in its motion caused by external forces. According to Newton's first law of motion, also known as the law of inertia, an object at rest will remain at rest, and an object in motion will continue moving with a constant velocity in a straight line unless acted upon by an external force. The mass of an object determines how strongly it resists changes to this state of motion.
For example, consider two objects with different masses, such as a small ball and a large ball. If both balls are initially at rest and the same force is applied to each, the larger ball will require more force to accelerate it compared to the smaller ball. This is because the larger ball has a greater mass and therefore more inertia.
Inertia is often referred to as the resistance to changes in motion, and mass is the fundamental property that quantifies this resistance. Hence, mass is the key factor that determines how much inertia an object possesses.