Thank you for the clarification

As Thalfon said, the Carbon 14 decays at a predictable rate.

So where does the starting quantity come from?

Radiation from space (probably cosmic rays) interacting with the atmosphere causes some of the atoms to become a different isotope. Plants integrate C14 into their sugars and cellulose, and when they are dead and buried by sediment, no more C14 is integrated. While buried, the plant matter is sheilded from further radiation, the C14 decays at a predictable rate, and we measure that. Meanwhile, underground carbon like coal and oil is also sheilded from radiation and doesn’t get converted to C14; instead any C14 decays into C12. Thus the problem with Carbon dating after the industrial revolution.