How many exclamation points is too many?
You should only use one exclamation mark.
In formal writing (such as writing essays and reports), it is improper to use more than one exclamation mark. Using more than one is seen as informal.
See these examples:
The ocean covers over 71% of the earth's surface!
The ocean covers over 71% of the earth's surface!!!!
In general, use exclamation marks sparingly - they can become weary with over-usage.