The Population of a graded coin is determined by the number of coins that have been graded in that specific grade by a reputable grading company.
Abbreviated to "pop" or "pops", so if someone is to refer to a coin being a "pop 12", this would mean that there are 12 coins graded in that specific grade by a specific grading company.
This population for instance, if the NCG has graded 100 MS 66 coins, the “pop” would be 100. This would only apply to the population of that coin graded by NGC and does not apply globally as this would be an impossible task to gather populations from all other grading companies from around the world.
Population of a coin does indeed change over time. Just because a coin has a low “pop” it may not stay that way for ever. Coins with the same grading may appear from time to time, therefore changing the population of that coin.
This is not to say that a high population of a coin means a low demand, there are coins currently with a high population and very high in demand.
We advise buyers and investors to be vigilant and not only rely on a sales pitch where a coin with a higher pop ie (MS-66) is a better investment than that of a (MS-62).
Reputable grading companies will have access to population figures.