You know there is no universal answer for that :^) As long as you are not seeing enough shops to keep you happy and shops you find interesting, you keep signing up. There is a list of 200+ companies in the first post of the first thread of the Mystery Shopping Company Discussion section. I understand there are a few things in there that may now be charging fees--if there is not a free way to shop the company offered, just move along. You do not need to be paying ANYBODY to shop.
The reality is that with 200+ I am signed up with, I work with only 30-40 of them per year because either they don't have work in my area or I find something objectionable about them (time to pay, rate of pay, silly pickiness of editors, etc. etc.). Because clients often change companies they are working with (especially at the end of the year) it is the better part of wisdom to be signed up with lots of companies so that you will know when one has just gotten a client in your area by email. And you will ALWAYS be looking for new companies to sign up with.
But you don't need to do it all at once. Sign up with a few each month and be ready to search their job boards during the last week of the month and the first week of the following month. If there is nothing in your area, there is a good chance they have no clients for you. So while you might check them for a few more months, mostly you will need to be adding more companies to check next round.