Yes, it is possible to CNC milling stainless steel that is very thin or very thick. However, there are some limitations to consider.

Thin stainless steel

Thinner stainless steel bends and warps more easily than thicker stainless steel, which makes machining it more challenging. There's also a chance of chatter, which can lead to a poorer surface finish. Thin stainless steel cannot be CNC milled except with a slow feed rate and a sharp end mill. You may also need to use a support fixture for additional protection against warping or bending.

Thick stainless steel

Thick stainless steel is more difficult to machine than thin stainless steel because it needs more force and power. Additionally, there's a greater chance that it will generate heat, which could cause the part to warp or shatter. For CNC milling thick stainless steel, you need a sturdy machine and a sturdy end mill. You may also need to apply coolant or lubricant to prevent the part from overheating.

Generally speaking, stainless steel that is less than 0.02 inches thick or more than 1 inch thick should be avoided being machined. Nonetheless, stainless steel of any thickness can be CNC milled using the appropriate tools and methods.