Amount Vs. Number

These two words work in the same way as “less” and “fewer”, referring respectively to collective and individual items. They are also often referred to as non-count and countable nouns.
 
The rules:

• “Amount” is a noun that is used in conjunction with non-count nouns. While these nouns can be measured, they cannot be counted. For example: Water
• “Number” is a noun that is used in conjunction with individual things that can be counted. For example: Cat

 
       AMOUNT: Collective (non-count) nouns                            NUMBER: Count noun

The error I see/hear the most often is when irregular plural nouns are mistaken for collective nouns.
Example:
There was a large amount of people at the concert – INCORRECT
There was a large number of people at the concert - CORRECT

Remember that we can count people, so "number" should be used.