Circulating supply is the number of coins (cryptocurrencies, tokens) in circulation.
It can be defined different ways – the key is that it’s defined the same across components of the equation of exchange.
For example, if circulating supply means “coins which moved within the past year,” the velocity, monetary base, and total purchases should also reference those specific coins.
Generally, it’s helpful to define circulating supply to include coins with a coin age less than one year, and exclude coins on-exchange, staked, and burned.
Read Why Market Cap Is A Bad Metric to learn why other measurements of supply are misleading for valuing cryptocurrency with the equation of exchange.