Bench grinder or hand grinder?
I have only a single old bench grinder, and it's not enough. You need at least three wheels and a wire brush. Plus a third set up for polishing.
For hand grinding, I have only one and it's not enough. At least one set up for grinding (preferably two with different grits), and another with a flap disk. That's three. Add one if you use a cutting wheel.
If you are including die grinders and Dremel-sized devices, wellllll...