As PCVolunteer said, it really does vary by position and, maybe even more importantly, by placement officer. PC is one agency/service for which any generalizations really can't actually be applied across the board.
And keep in mind that PC Response (known as PCRVs) and being a PC Volunteer (PCVs) are two very different things in many ways, and separate and distinct processes. So generalizations across those are pretty much impossible!
Anyway, when I went through I was told about 2 per position were interviewed, give or take.