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Credit Karma's scores and "recommendations" like that are jokes. Of course Credit Karma wants you to have 20 credit cards - they hope you get them through their affiliate links!

Note that the "recommended" lines of credit is lifetime credit and includes things beyond credit cards.

I mention this due to the context of this thread being "US residents have lots of credit cards", not because I think you misunderstand.

For the record, I view that as gaming the credit cards. That is an oddity of the way that credit reporting agencies work that using 5 credit cards is perceived as better than spending the same amount on 1 card.