Stupid Question Alert

"Rare" is a relative term. Each country has its own scale of rarity. Here in Australia, for example, Australasian tradesman tokens are graded on a scale of 1 to 10, originally used by the early cataloguers of the series, where R1 is most common and R10 is overwhelmingly rare (unique or almost unique). But, since even the most common of these tokens have survival rates less than 10,000, all of them would be considered "rare" when compared to, say, American coins.

It's perhaps best to consider "rare" as a qualitative term, rather than quantitative. Since rarity is only loosely tied to value, it matters less than it ought to.

"Current population" is a semi-quantifiable equivalent of "rarity". Current population = original mintage X survival factor. The original mintage is usually knowable (except for mediaeval and ancient coins, for which few if any records remain). The survival factor - essentially, the probability that a coin will survive being melted down, buried, locked away in museums, lost at sea or otherwise become unavailable to collectors - usually has to be guessed at, as reliable statistics on such things are impossible to collate.

Collectors themselves are part of the uncertainty, as a coin in a private collection is "unavailable", but only temporarily. Investors are a much more predictable bunch, but the proportion of collectors to investors in any given coin series is also difficult to determine. There's also a continuum between "collector" and "investor", where someone is "a bit of both".

Coins generally become "more rare" over time as their numbers gradually dwindle due to attrition, though occasionally some event can make coins become "less rare". The release of the GSA hoard of Morgan dollars are perhaps the best-known event that created a sudden decrease in rarity for certain date-mintmark combinations in that series. It should also be noted that this "attrition rate" is different for each coin. "COmmon" comins become comparatively scarcer over time, as they are more likely to be discarded and melted than a coin known to be scarce.