What are you shooting at it? If you're shooting 5.56 don't shoot M855/SS109.
Min is probably 3/8in, I use 1/2in, but more important than thickness is the type of steel; you'll need AR500 so that it won't crater.
When a bullet hits steel it disintegrates into may very small pieces and a few larger pieces usually pieces of jacket. These fragments will exit the steel plate at a 20° angle from the surface. While 20° is safe to shooter and bystander, if the surface starts to crater things get ugly. When the craters form they can have very steep walls, so the next round will leave the surface at 20° and from a crater this could come strait back to you causing mild to serious injuries.
These are some targets I made, and they work well, but the IDPA/IPSC style stands didn't hold them well so I've since built new stands.
Good luck