This is a very hard question - my Priest told me that we shouldn't think in numbers.
However, most estimates are between 250 million to 350 million. Other estimates are much higher including people who are Orthodox on paper but are not actively involved in the Church.
Also, this number is steadily increasing in many former Communist countries where people are returning back to the Church. There are also many non-Orthodox parishes accepting the Orthodox faith throughout lands that are not traditionally Orthodox.
There are also many millions of Non-Chaledonian Oriental Orthodox Christians, with the largest number in Ethiopia.