If you want to make this a Stone Cobblestone and Smooth Stone farm just add more furnaces to smelt the Stone into Smooth Stone. You will also need a fuel source like Coal or Blaze Rods.

Just make sure you dont go too far away from the farm because the chunks might unload and the farm could stop working. Even moving 80 blocks away can cause problems.

If you leave your PC running while you go do something else it should be fine as long as you stay near the farm. I tested it for 12 hours straight and it worked perfectly without any problems.

Fix the grammar and spellingClarify the furnace expansion steps