Does the introduction of credit card affects Money Velocity?
Credit cards are a way of deferring payment in the form of inter-temporal substitution; it helps reduce the household's liquidity constraint that may prevent it from consumption smoothing as much as they would like if their transitory income was much more spread over the household's lifetime.

You are right that the amount of money doesn't change. Loans require giving away money, but credit cards are a way of creating debt, so banks do not need to hold more cash to give a line of credit like you suggest.