(Two, I think.)
The big issue with reactor numbers would be simplicity of maintenance vs. redundancy in case of catastrophic damage. A single reactor would be a single failure point, which is actually good when you don't want to worry about eight
points that might fail at any time, or even at the same time. However, if you can trust your eight reactors not to fail too often, you might appreciate the fact that any four of them might go down and you'd still have half your power left; with a single reactor, you'd have either all or nothing.
Of course, if you can really trust your eight reactors, you might wish to build a fourfold-uprated model so that you could have just two things to worry about, and still enjoy redundancy. And three powerplants, each of which alone would be able to move the ship at a crawling pace at least, would indeed sound appealing for the TOS ship.
Then again, we do know that the TOS ship tended to fail at a single point. The dilithium focus was the weak point, almost completely disabling the ship in "Elaan of Troyius" when sabotaged. From this we might deduce that the man-tended parts in the engineering hull are the important bits, and whatever may lie in the nacelles can't get the job done in case of catastrophic failures, so it might just as well not be there at all.