Personally I'd like to see at least 12" of clearance.

If you provide vents in the cabinet, that should be enough if sized adequately. As guidance for size, you can use the total free area of the electronics' vents. Anything much smaller and you will need to go with a fan assist. This also makes the assumption that your front panel door is fairly tight. You will get some air leakage through it to add to the natural ventilation and give some flexibility on vent size..