There is no limit to the number of times that you can take the the GCE-Level examinations as a private candidate. You may register for a GCE-Level examination as a private candidate during the annual Private Candidates’ Registration Exercise.
As a private candidate, you may decide on which subjects you wish to register for, based on the list of available subjects for a given GCE-Level examination year, and the maximum number of subjects that a private candidate is allowed to register for, during each GCE-Level examination sitting.