Arithmetic on today's supply, not a forecast. It assumes nothing about whether Zcash could attract that much money.
A coin's market capitalisation is its price multiplied by the number of coins in circulation. Zcash has 16,903,674 ZEC in circulation at $1,141.10 each, which is $19.29B. Holding that supply still and putting Ethereum's $307.54B in its place gives $18,193.86 per ZEC.
Zcash would have to take on $288.25B of new money to get there. A 15.9x move is large: it happens, and it usually needs the whole market to grow instead of one coin to take share from another.
Supply is held still, and in practice it moves: coins are minted, burned and unlocked, and every one of those changes the price a given market capitalisation implies. The figure also says nothing about whether the money exists to buy ZEC at that level, which is the part that actually decides a price.
$18,193.86 per ZEC. That is Ethereum's current market capitalisation of $307.54B divided across the 16,903,674 ZEC in circulation.
15.9x. Zcash trades at $1,141.10 today and Ethereum's market cap is $307.54B against Zcash's $19.29B.
No. It is one division: Ethereum's market capitalisation spread over the supply of ZEC that exists today. It assumes nothing about whether Zcash could attract that much money, and it holds the supply still when in practice supply changes.
It sets it. Market cap is price times circulating supply, so the same $307.54B of market cap spread over more coins gives a lower price per coin. This uses the 16,903,674 ZEC in circulation now.