I would say to do a couple layers for a home gym, or if you are running an oly gym.
Residential concrete is not the same as commercial in a lot of areas, your home will need more protection.
If you are using something like hi-temps, you might get away with just mats and a piece of wood with moderate usage and moderate weight.
If you are using a competition style plate, have higher lifts, or will do daily training on it, take the time to build a proper platform
I would say it is like a seatbelt, you might never need it, but the one time you get in an accident you would be damn happy you were wearing it.
You might never damage your floor, but if you did you would be damn sorry you didn't spend the extra $$ for another couple sheets of 3/4'' osb
So for a home gym, spend the money on plywood instead of foundation repair.