For a stablecoin, market cap ≈ circulating supply × peg price ≈ circulating supply.
Market capitalization for a stablecoin is calculated as circulating supply × market price. Because a well-behaved stablecoin trades at (or very near) its peg, market cap is effectively equal to circulating supply. For non-USD stablecoins, market cap is usually expressed in USD-equivalent by applying the FX rate. Note this is different from valuation for a volatile cryptoasset, where price × supply can swing hugely.