Total Supply is the number of tokens currently in circulation in this sToken market. It is part of the EIP-20 interface of the sToken contract.
SErc20 / SEther
function totalSupply() returns (uint)
RETURN: The total number of tokens in circulation for the market.
SErc20 sToken = SToken(0x3FDA...);
uint tokens = sToken.totalSupply();
const sToken = SEther.at(0x3FDB...);
const tokens = (await sToken.methods.totalSupply().call());