A lot of True Orthodox seem to overestimate their numbers.
In 1999, it was estimated that "[t]here are probably over one million Old Calendarists in Romania, somewhat fewer in Greece, and considerably fewer in Bulgaria, Cyprus, and the [Eastern Orthodox] diaspora." The Blackwell Dictionary of Eastern Christianity. Oxford, UK: Blackwell Publishing Ltd. p. 499
All told, there are probably only two to three million True Orthodox worldwide.