Well whole thread is about doing material that you can manipulate from unreal and apply different material parts to faces.
You need to decide if unreal does heavy work of stitching together material for those boxes, or you do it in 3d application.
IMO you should get blender, find out how to make sub materials there, and do your box proper way. Its just box, perfect example to learn very basic stuff.
If you don’t do it now, next time you will have problem with gun, chair or something else that you cannot find ready to use. Or if you ever find shape you need it will
have multiple errors that need to be fixed for unreal. So do this thing now and learn new stuff.
Your problem would be very trivial if you could make 16 boxes, each for different number. Then there would be no need for complicated materials etc.