He can grow one towo or three extra hands all he likes. It will avail him not since at the end of the day he will always have only two real hands. Which was why Natsu is more than capable of taking the savage course of biting into that clawed hand and chombing down hard. Only Gray will then notice the swirling energies contained inside the cavern of his mouth and the look in Natsu's eyes that just screams 'GOTCHA!"