Bitcoins are created at a decreasing and predictable rate. They are created in a process called mining, which comes down to adding transaction records to Bitcoin's public ledger of past transactions. Every four years the mining reward drops by half. When the initial creator of Bitcoin released the idea in 2009, he wrote:

"Total circulation will be 21,000,000 coins. It'll be distributed
to network nodes when they make blocks, with the amount cut in half
every 4 years.

first 4 years: 10,500,000 coins
next 4 years: 5,250,000 coins
next 4 years: 2,625,000 coins
next 4 years: 1,312,500 coins"

It is moving a little faster than it was originally planned but it is not too far off.  In 2015 there will be ~15,750,000 bitcoins.  By 2030 bitcoins will be very scarce. The predicted graph is:

Total bitcoins in circulation (projected)