Prime XBT supports 5 different cryptocurrencies - namely, Bitcoin, Ethereum, EOS, Ripple, and Litecoin. While the number of supported cryptos is surely very small, it should suffice, seeing as it involves all of the 'mainstream' and most well-known crypto coins. If you'd like to visit an exchange that would operate in the US, however, and that would offer more crypto choices, one of the best alternatives is going to be Coinabase.