More layers = more protection
If you think of Kevlar "soaking up" the energy of a bullet, it's fairly obvious that a greater thickness of Kevlar—more layers of the material bonded together—will give more protection. As you can see from this chart, the more layers you have, the faster you need to fire a bullet to get it to penetrate through Kevlar armor. In other words, if you want to protect soldiers against high-velocity rifle bullets, you're going to need much thicker armor than if you simply want to protect police officers against handgun bullets, which have lower velocity and less kinetic energy.