short answer: market cap
lets say you have 100 apples and put all in circulation. now if 1 apple is worth $1 then the true market capitalization of apples is $100 (supply * price).
now lets say you have 100 apples but only put 10 of them in circulation. now if 1 apple is worth $1 you still get the same market capitalization of $100 but it is fake because the real market cap is $10
Is this really true? I used to know that Circulating Supply determines the market cap. Look at Zcash-
https://coinmarketcap.com/currencies/zcash/The maximum supply of Zcash is 21,000,000 ZEC but the circulating supply is 9,110,906 ZEC. As per current price, Circulating Supply * Current Price = $557,634,432 which is the current Market Cap of Zcash. I don't know where you get this or what I'm missing.
you are still correct, the "circulating" supply is used in calculation of market cap not the total supply. you didn't need to go that far either, you can look at the first in the list (bitcoin) and see the circulating supply is used instead of the 21 million cap.
BUT the problem is with fake circulating supply. take one of the biggest shitcoins called XRP for example. a huge percentage of the supply they claimed to be "circulating" is instead locked up in their own wallets. so it is unreal. a lot of others like ethereum are also like this with huge premines.