When and how should I use multiple exclamation marks?

Never.

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:

pwnd!!!!11!!111!one!!eleven!!!!

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.

"Keep your exclamation points under control. You are allowed no more than two or three per 100,000 words of prose. If you have a knack of playing with exclaimers the way Tom Wolfe does, you can throw them in by the handful." (Elmore Leonard)

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.

No. Stop. Period. End. That's it.
Do all those extras add to the meaning that I say No and no more than that, grammatically?
Same with exclamation marks, I guess. Or else it would undermine the power of the Wonder mark.