If you want the switches to clip in, 1.6mm is about the upper limit. Alps switches actually work better with a 1.6mm plate than cherry switches do. With cherry switches, even 16 gauge steel is sometimes too thick for them to clip properly. For both Cherry and Alps switches, my own recommendation would be to stick to 18 or 17 gauge steel.
With Alps switches and a 1.6mm thick plate, you want to slightly adjust your cutout shapes so you avoid crushing the little plastic “wing” clips. If you plate is too thick and too tight, you’ll end up breaking a lot of them. If you’re soldering everything down to a PCB this isn’t the end of the world, but it can prevent you from desoldering those switches and using them in another context.
You can make a plate as thick as you want, if you adjust your cutout shape to entirely avoid the plastic clips, and just make the plate tight enough to the dimensions of the switch that it’ll be held in by friction. If you want to do that though, you need to make sure your cutting process is very precise and locally accurate, since if you make it too tight you’ll be stuck filing things down by hand to get your switches in, and if you make it too loose they won’t stay. Again, if you have a PCB then it’s not quite as big a deal.
If you want to layer acrylic + steel below, just cut the normal Alps cutouts in the acrylic (which you should be 1.5 mm or 1/16" thick), and then cut slightly larger holes in the steel in a shape which avoids smashing the clips. That should work pretty well.
Alternately, you could use as thick a plate as you like in whatever material, and use a CNC mill to cut a little ledge from the bottom side of the plate for the clips to snap into, in effect making the plate only 1.5mm thick at certain spots where you need, and as thick as you like for the rest.
