Hi Ash Standard,
Even though you’ve gotten a lot of information about spacing of support members and nails versus sheet rock screws I think you were asking what would be a safe overhang using 3/4 inch plywood on top of your existing table. In my career I did consulting for a kitchen cabinet manufacturer and a kitchen design outfit. I would suggest whether you use 3/4 inch plywood or Advantech, only have a maximum of 12 inch overhang.
Also during my career I did plywood research and only God knows how consistent the inner plies are and how many voids are in them. I found by far the best plywood on the market is that made by Plum Creek.
Jim Lawson