On a small phunnel 2 cocos are enough heat but the issue is heat distribution. 3 heat sources work better than 2 and you can get by with 3 halves actually. Personally I just use 3 whole charcoals because it gets the bowl up to temperature faster and the session will last longer than with halves. Just be careful not to overheat the bowl at first or place them on their sides.
Actually, the real reason is because I'm just too lazy to split the charcoals