This can be changed per Activation (e.g. per Serial Number) or per Activation Key within the Data Management Tab, Option View (Modify an Activation). The main purpose is to allow another activation for a specific user.
The default number of activations (NoOfAllowedEntryOccur) / Activation Keys (NoOfAllowedKeyOccur) for a specific project (ProjectID) is defined in the project.config. file
If you want to allow more activations for a specific Serial Number or Activation Key in advance (before handing out the key) How can I specify the number of times a key can be used?
See also
Maximum Key / ID usage
Effective Key / ID usage