Well, many ponies may believe that that is how you should phrase that question, but really the question is "why would i put a black hole on a pin?" Heh heh, well in answer to your original question, there are an infinite amount of black holes that can fit onto a pin, you see, they are infinite in size on a plane. Black holes aren't exactly 3-D in shape. Take a triangle, we see a triangle, but what actually exists is a never ending prism, and our eyes can't exactly see that. So technically we can fit any amount you want onto a pin, just dont suck in any major planetary systems!