"Third-party preparers register once, and not for every client they represent."

And:

https://www.ssa.gov/employer/bsorahelp.htm#19

"Third Party Filer Questions:

Q 19. I am a third party filer (accountant, CPA etc.). Do I need a User ID for each company I am doing business for?
A19. No, third party filers need to register only once. Register using your company's EIN, or indicate you are self-employed. On the Registration form you should select the box indicating "I am a 3rd party submitter registering to do business on behalf of another company"."