A coin’s population is the number of coins that exist in a particular grade. Populations are the main distinction for a certified coin’s overall rarity.
Contained within the overall mintage of a certified coin is the coin’s population. The higher-grade specimens of a coin tend to be less populous because of a number of factors such as the difficulty of achieving a perfect strike or the degradation of coins that haven’t been preserved.
You can look to a coin’s population to see just how few of them are available in the world. There is an important distinction even between the profit potential of a Proof-69 versus a Proof-70. The population of a coin in a particular grade can be as few as zero with the ideal scenario for a collector being that they own a coin with a population of just one coin for that grade.
The best way to own certified coins in general is to own all of the years and denominations of issue for a coin program (known as a date run) in the highest grade possible. This covers all bases and maximizes the potential for the coins to appreciate.