In the older days of layout building, the recommendation was for 1" plywood for O-scale and S-scale, and 3/4" for HO. N-scale wasn't around then but I bet the recommendation would be 1/2" for it. I remember reading this in a MR from the '40's-50's, on the construction of the Tuxedo Junction. This was an entire series over several years on building a large model railroad, by 2 guys and was being built in a barn IIRC. It was heavily influenced by the PRR & NYC around NY.
But since the mid 1960's the plywood thickness has become a moot point. As long as the plywood is supported properly, any thickness could be used for any scale. I had a friend that had a O-scale layout in a 3 car garage, and in places he used 1/4" plywood with a 1x2 glued on edge under it to supply support. We had no problem with it sagging or getting distorted. He had a set of All Nation F-3's in an A-B-A configuration, and with the extra weight installed into the bodies, and all units powered, this set weighted close to 30lbs! The plywood acted like the locos and 50 coal hoppers weren't even there.