bitcoin gold is a fork of bitcoin, it means they copied everything from bitcoin, the code with a little change and also all the blocks that have been mined so far (until 24th last month).
because of that about 16 million coins are already mined (the coins mined on bitcoin originally) and the cap which is 21 million is also a copy of bitcoin so there are about 5 million more coins remain to be mined. just like bitcoin.
that is just mining though. the circulating supply is the same as bitcoin because as i said BTG is a copy of bitcoin with some changes.
these changes were two things which made it an altcoin:
1. the mining from PoW to EquiHash
2. added a large premine for the developers. 200,000 BTG!
Thank you a lot..Only 5 millions of coins for mining. Ok, I will mine 5 or 10 coins. After that, I will change to other coins.