Typically, only one exclamation mark is used. In informal writing, people often use one (or many) to emphasize their excitement.
For example: “Congratulations!!! You stood first in the class!”.
Multiple exclamation marks are just for added emphasis. When we need to add more emphasis on an exclamatory word to express emotions in text, we generally use multiple exclamation marks.
I do not believe it is strictly grammatically correct.