The requirements are incomplete and don't constrain the problem. You have to say the maximum temperature allowed on the cabinet. From what you have written, any size and number of holes will do. Many large holes will result in a low maximum temperature and a few small holes will result in a high maximum temperature.

I assume you want this to be cooling by natural convection. You have to say what the air temperature is in the room. You will also need to define the walls of the object inside the cabinet that are dissipating the heat. The object inside must be supported by some structure that connects to the cabinet. Is that structure insulated or does some of the heat conduct to the cabinet wall? What boundary condition will you use on the Bottom?  Is the cabinet sitting on a table or is it on four legs and have exposure to the outside air on the bottom?