This is something you should be able to find out using the audit trail. Of course is has to be turned on, which by default is not. The audit trail is accessible only limited from Alfresco itself. It's ready to be queried from the database (alf_audit_*).
Since you only need the creator property, this might also be something to do with CMIS queries. I've seen COUNT statements work there. Not sure if this covers your needs.
Koen