How many times do you knock when you come in the room?

How many times do you knock when you come in the room? I heard that There is "protocol manners (International Standard manner)" about the times you knock the room when coming in. "Twice" is when you come in the toilet=so it is bad manners. "Three times"=when you come in the... show more How many times do you knock when you come in the room?
I heard that There is "protocol manners (International Standard manner)"
about the times you knock the room when coming in.

"Twice" is when you come in the toilet=so it is bad manners.
"Three times"=when you come in the husband or wife or friend's room.
"Four times"=humbles way of knock (you come in the superior or professor's room).
<source>http://netafull.net/life/035342.html (*Japanese website)

I wonder such way of manners is truly exist because I have never heard such manner expect this website. So would you tell me your country's manners of knocking?
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