For example would a key with 15 random letters be enough to protect a 52 letters plain text?

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    $\begingroup$ Did you hear about the perfect secrecy and C. Shannon $\endgroup$ – kelalaka Dec 28 '19 at 8:31
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    $\begingroup$ I'm voting to close this question as off-topic because of a complete lack of even the most cursory research. $\endgroup$ – forest Jan 1 at 10:33

This is well known. The pad must be made of uniformly chosen independent letters and be as long as the plaintext.

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