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It depends on the card's rewards program. As a rule, general travel rewards credit cards have a redemption ratio of 1 mile = 1 penny. So, you will basically need to multiply the cost of your ticket by 100 to see how many miles you will need. Thus, if an airline ticket costs $265, you will need 26,500 miles to cover the cost.
If you are using frequent flyer miles, then they work a bit differently. Most airlines have redemption charts where you can see how many miles you need for domestic and international flights. Note that those redemptions do not include taxes and fees, they are paid additionally in cash when you book your award ticket.