Now, I never do this, but in some few cases I have seen people use multiple exclamation (or question) marks like this:
Is that grammatically correct? (Or just okay). In case it is, how many marks are grammatically allowed/accepted?
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'Multiple exclamation marks,' he went on, shaking his head, 'are a sure sign of a diseased mind.'
-- Eric, Terry Pratchett
More on this subject on the Discworld and Pratchett Wiki.
It's just for added emphasis. I do not believe it is strictly grammatically correct, but then using ALLCAPS is not, but people do that too, emphasis once again.
It's fine in informal communication, email, poetry, and advertising headlines. Three would be most common. Two, four, or more is rarer.
It's never "officially" correct. The people who write books about how punctuation should be used in English tolerate only two levels of enthusiasm: not enthusiastic and enthusiastic. The idea that somebody might be very enthusiastic is too alarming to contemplate.
At least not for grammatical purposes. More than one exclamation mark doesn't have any meaning. An exclamation doesn't get more "exclamationy" by more marks.
Of course, you could still use them, but the interpretation would be entirely up to the reader. Use of punctuation that doesn't have any grounds in grammar would be more like decoration.
I've seen people invent their own style of punctuation,,,like tripple commas,,,but that of course doesn't have any meaning either,,,it mostly makes the person look unstable...
Multiple exclamation marks are sometimes seen in leetspeek, often intermixed with intentional mistakes in the form of 1, one or eleven:
One is enough to convey your feelings in conventional English but people seem to be taking it symbolically to mark the full intensity of their emotion and playing with it sometimes putting more than three exclamatory marks for a simple wish; However, we do not take care of it when it comes to conversation.
Punctuation isn’t grammar. Where and how often you use exclamation marks and how many you use at any one time is a matter of stylistic judgement. There will rarely be a need for any at all in an academic paper. If, on the other hand, you’re writing the captions and speech bubbles for a comic strip, let your maxim be, the more the better.