18/8 is a commonly used stainless steel term and contains 18% chromium and 8% nickel.
This steel is also known as 304 in America, while in Europe it is 1.4301.
Some cutlery uses '18/10' which is purely a marketing ploy as the "10" does not indicate an actual higher Nickel content.
So instead of drinking hot coffee from a plastic bag invest in a stainless steel bottle for the health benefits.