If you're using iron and no bumpers, 1 layer of horse stall mat may not be enough. The issue is stress over time. Stall mat will deform slightly so the force isn't spread out so much. The force is only distributed across the line the plates contact the floor (which is small so PSI is much higher).
Strongman types (like myself) have to really be careful when it comes to stuff like atlas stones were the stone (maybe only 300#) will fall and because the point of contact is so small you can crack concrete over time with even 2 mats. I've heard of concrete cracking with 3....but only hear say.
A layer of plywood or OSB does alot. If it's in a basement, get plywood. OSB can become warped over time absorbing moisture from the cold/Damp concrete floor of a basement