ISTR that it is estimated that there are over 1 million project managers with the PRINCE2 certification, but can someone point me to an accurate reference for this and/or any other certifications such as PMP?
PMI publishes "PMI Today", which displays the number of Active certification holders. As of August 2015 there are 655,576 PMPs, and 28,826 CAPMs. Another up-and-coming certification is the Agile Certified practitioner PMI-ACP which has 8,870. The other 5 certification streams have less than 4k.
All told a little under 700k. Keep in mind that some people may hold multiple certifications, and there are other organizations who do PM certs too.