That means that there only another 8 million to be mined over the next 140 years or so until we reach the total maximum of 21,000,000.
The mining process used to mint new bitcoins and secure the network originally paid out 50 bitcoins for each block but that bitcoin reward halves every 21,0000 blocks. Currently the bitcoin reward is 25 bitcoin every block making it so block 310,001 contained the 13th million bitcoin to enter circulation.
At the current price of somewhere about \$622 per bitcoin that puts the market cap at \$8,086,000,000 (or about 8 billion) US Dollars.
Because of the anonymity features of bitcoin here is no way of actually knowing how many of those 13 million are actually in circulation or who owns them. It is supposed that Bitcoin inventor Satoshi Nakamoto may have close to 1 million, there are many threads on the bitcoin community site Bitcoin Talk about people loosing bitcoins by loosing their private key's but in the end, a total amount of bitcoins in circulation is just speculation.
However, we can now say that there are 13 million!