There is no firm answer to that. We've seen at least four canonical Pyramid Heads based on appearances in SH2 and Homecoming but that's the type of question that Konami and the masterminds behind Silent Hill would never provide a solid answer to. If we knew there were only five of them, it would diminish the fear. A good villain has an infinite sense to them. Like how it doesn't matter how many movies Jason is in, he just won't die or go away. While there is a reference to a "Red Devil" in Silent Hill 4, that was confirmed to not be Pyramid Head so as far as we know there are anywhere from four to four thousand of them.