how do I know when and how much my apple card payment is?

The payment due date is ALWAYS the last day of the calendar month.

The minimum due and full balance amounts will become available in your Wallet on the 1st two or three days of the new calendar month.

See more details here:

How to make Apple Card payments - Apple Support

Chattanoogan is spot on with his post. Just to add an additional resource for the future you can access your Apple Card statements online.

If you do not have access to your required Apple device iPhone/iPad then use a computer and use you Apple ID to log into card.apple.com and download your most recent statement from the list in the upper left corner (tap the 3 horizontal lines) and view statement. All the information you need is there, including due dates.