Since you don't have any predators I would order 10 of the medium size what will run you about $100 or you can even double that. It'll just take them a little longer to get going for the smaller size and initial quantity. They won't appear to be doing anything at all at first and then all of a sudden you're going to see them everywhere. When you get them, put them in at night in low-flow area.
Thank any recommendations on how many