how do I know which credit card was exposed, as I have more than 1?

For security and privacy reasons, Mozilla Monitor only knows that a data breach occurred at a specific site and what types of data were stolen (e.g., "Email, Passwords, Credit cards"). They cannot tell you which specific card was exposed because they don't store that data. Look at the Mozilla Monitor alert. It will list the name of the company, website, or service from that data breach and check which of your credit cards you used on that specific website (if you remember). More info [[Mozilla Monitor - Frequently asked questions]]