Earlier quoted context omitted.
It's full size they just use 27+ of them on the heavy lift version of their rocket, and 40+ on the proposed rocket for lifting the Mars ships off of Earth.
Falcon Heavy will use 27 Merlin engines, not Raptors. The Merlin is already being flown in Falcon 9, and they're producing dozens per year. The Heavy is basically a Falcon 9 with two more Falcon 9 first stage cores strapped on the side as boosters. Though I think there is talk of moving all Falcon to the Raptors eventually.
Because that would decrease the redundancy, right? With Falcon-9 you would lose 2/9 of thrust if you turn off the mirrored engine in the best case. In a Falcon-3 scenario, you would lose at least 1/3 for middle engine failure or 2/3 for a side engine failure.