Nobody can tell you for certain what the total amount of mBTC is, that are in circulation. We know how many have been mined and we know some of them are owned by Satoshi and it has never been touched, so they are definitely not in circulation. We also know how many were "burned" < send to addresses that cannot be spent again >
Your best bet would be to take the total bitcoins mined and divide them by mBTC.
https://99bitcoins.com/23-bitcoins-mined-13-may-lost/