While there are currently around 18.3 million bitcoin in circulation, more are being created each day through a process called "mining."
There will never be more than 21 million bitcoin that will ever be created. This means that over 85% of all bitcoin have already been mined and are in circulation!
Over time, fewer bitcoin can be found through the mining process. About every four years, the reward that people receive for mining bitcoins is cut in half. Due to this supply reduction over time, there will eventually be virtually no more bitcoin to be mined. This is expected to happen around the year 2140.