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