How many best friends can you have?

As many as you want. But be sure you truly know them and that they are not living a lie. Be sure to pick good ones.

A person can have as many best friends as they want to.

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* Most technically, you could only have 1 "best" because the superlative indicates that none of the others could be "the best." However, in many uses "the best" has come to mean a group of the highest prominence (e.g. best new shows). And even if applied literally, you could still have a "best male friend" and "best female friend" and even "best adult friend."