I'd add just one at the most. The rear brake is 90% useless, so why have two levers for it?
Next question would be do you want thumbies as well? If not, and you are using bar ends, brifters or downtube shifter, you won't really get the use out of the top mounted levers you tink you will.
I set up my bars so the hoods are the comfort zone, the tops are for tailwinds, and the drops are for headwinds. That's an oversimplification, but just so long as you understand the tops should be set higher on a raod bike than on a flat bar MTB.
I'm running two brakes on the front, and I will be running the standard road levers, and a Paul e-lever on the tops, as soon as I get rigged up with the lever.