Three times

Narrated Anas:Whenever the Prophet ( Peace Be Upon Him ) spoke a sentence (said a thing), he used to repeat it thrice so that the people could understand it properly from him and whenever he asked permission to enter, (he knocked the door) thrice with greeting. ( Bukhari)
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O you who believe! Enter not houses other than your own, until you have asked permission and greeted those in them; that is better for you, in order that you may remember.
And if you find no one therein, still enter not until permission has been given. And if you are asked to go back, go back, for it is purer for you. And Allah is All-Knower of what you do.  [Surah An-Nur: 27-28]