lets say you launch your coins:

you premine all 100 out of 100 coins so 100/100

you decide to only give out 40 of the 100 coins

and then 40 coins can be used for circulating supply
even though there is 100 coins of total supply

try to look into ripple its craaaazy how much total supply they have compared to their circulating supply
imo (against everything crypto stands for since they can control freely the prices of the coin (more or less))

the market cap can be calculated if you multiply the circulating supply by its value