market capitalization is just a simple number that is calculated by multiplication of price and available supply.
now with altcoins their supply can be manipulated, for example Ripple has a ridiculously high supply of 100 billion coins so with a small price its market cap is a big number.

now if you want to speculate how much the market cap is going to be you need to speculate how much price is going to be.

price will be greatly affected by the supply. for example when there is 100 billion coins available the price drops hard like in the case of Dogecoin which is ~100 satoshi and less.
but when the coin has a fake supply but its circulating supply is small the price can go higher because there isn't the same sell pressure.
for example coins like XRP and ETH that are on top have a massive supply but it is not all available to everyone. they are stored away out of reach and just affect the number known as market cap to stay on top of the ranking.