The supply is capped at 21 million in purpose for it to be sort of deflationary; not like most fiat whereas the value of that certain currency goes lower and lower every year/every few years due to the government printing more and more money.
Why 21 million you say? Here's a good explanation:
Here's a mathematical explanation:
Calculate the number of blocks per 4 year cycle:
6 blocks per hour
* 24 hours per day
* 365 days per year
* 4 years per cycle
= 210,240
~= 210,000
Sum all the block reward sizes:
50 + 25 + 12.5 + 6.25 + 3.125 + ... = 100
Multiply the two:
210,000 * 100 = 21 million.
Economically, because the currency is effectively infinitely divisible, then the precise amount doesn't matter, as long as the limit remains fixed.
source:
https://bitcoin.stackexchange.com/questions/8439/why-was-21-million-picked-as-the-number-of-bitcoins-to-be-created