The amount of bitcoin anyone can mine is fixed to a certain amount. This amount gets halved every 4 years (this is known as the halving).
2008-2012 50 BTC/10 minutes10 minutes
2012-2016 25 BTC/
2016-2020 12.5 BTC/10 minutes10 minutes
2020-2024 6.25 BTC/
2024-2028 3.125 BTC/~10 minutes
This continues unt the year 2140.