Each fan is probably going to use like 3 to 5 W, so I wouldn't worry about the power added. The air circulation from the fans will probably greatly outweigh any heat added. More fans doesn't necessarily mean better tho. You should try to position intake and exhaust fans so you create the most flow. If possible try to position them so they don't conflict with your CPU and power supply fans. I think the main trade-off with having more air circulation is the potential for more dust accumulating. It's nice that the case has filters (although they will decrease the flow a little).
You probably won't be able to balance intake and exhaust perfectly, and you'll end up with positive or negative pressure inside the case. I think it might be a little better to have slightly negative pressure (i.e., vacuum, or slightly more exhaust) - that way you won't have lingering air with residual heat. In my case I have one intake fan in the front, one exhaust fan in the back, and another exhaust fan on the top. Managing the wires in the case will also help the air flow.