訳:He has a credit card.
You can do so, if and only if the hearer/reader recognizes the cards as specific credit cards and distinguishes the credit cards from other credit cards. However, I suppose this interpretation does not match the given Japanese sentence.
Thank a lot.
There are people who use the credit cards around me. They almost have multiple credit cards. There is no one who has only one card. Therefore I wrote the question because plural cards are natural in my mind.
Could you tell me what does not match?
Does that mean that Duo's answer is 'common sense' in English? Like 'At least he has a (one) card'? It is important that he has a card or not. The number of the cards are not need?
The core issue is the definite article, not plural/singular. "He has credit cards" would be accepted as a correct alternative, while "He has the credit cards" not. The latter suggests that he has specific credit cards which you (hearer/reader) can distinguish from other credit cards. One of the corresponding Japanese sentences would be "彼はそのクレジットカードを持っている".
For the same reason, the expression "There are people who use the credit cards around me" in your post should be "There are people who use credit cards around me". Because those cards would not be specific ones.
You do not need to worry about it, I am happy to share time with you. Pleasant weekend.
Thank you for reading my odd English. (I have learned the word 'odd' here.) See you somewhere in Duo!
Don't be afraid to make mistakes. Practice makes perfect. Good learning.