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