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What mix of credit lines and loans is optimal for my credit score?
I think you are interpreting their recommended numbers incorrectly.  They are not suggesting that you get 13-21 credit cards, they are saying that your score could get 13-21 points higher based on having a large number of credit cards and loans. Unfortunately, the exact formula for calculating your credit score is not known, so its hard to directly answer the question.  But I wouldn't go opening 22+ credit cards just to get this part of the number higher!