Mine also came with two - but I'm talking about the priming lever arm. It's my understanding that you can buy the Saftey Prime seperately for each, but I bought them as a pair.
If you have the spare ratchet that came with the press, you don't need another. I'm still on my first with about 10,000 rounds loaded. You won't destroy a ratchet as long as you remember to raise the ram slightly before removing or installing a turret. As the ram comes down, the ratchet is "engaged" so obviously it would also be at the bottom of the stroke. It comes out of this condition as the ram rises which is evident by the fact that the turret doesn't turn on the ram upstroke. You definately don't need another housing as all it does is contain the ratchet - there is no adjustment to it at all and it doesn't make any sense to tighten it beyond what is necessary to hold it in place to the cut out on the back of the ram.