I don't think it is the number of badges you get. I believe it is the number of points. You get 20 points for every shop you do but you lose 30 points for every shop that you take and then give back and don't complete. You also lose points for declined shops. So if you take shops and then unassign from them a lot you will lose a lot of points.
Here is a quick way that you can check. First no you will not be at rank one forever. As Wendy indicated, each successive shop awards 20 points toward your next rank. You can check your progression by doing the following: login to I secret shop, and then look at your rank badge or reputation badge at the very top of the page. You will see your badge along with a bar that will have a color line and a percentage. Percentage number is where you stand at rank one. Each successive shop will increase the percentage number until it reaches 100%. After that you will see rank 2. At each level it takes longer for the percentages to reach 100%, FYI, because you have to do more shops at each rank to reach 100%. There are a total of 10 ranks. If you notice for Wendy she is at rank six. I am at rank 4. So keep plugging along and you'll move up the ranks. Hope that helps