amount / number
How much of a mass versus how many of a count.
Use amount with mass nouns: amount of time, amount of water, amount of traffic. Use number with count nouns: number of hours, number of gallons, number of cars. Do not use amount of people or amount of errors. A number of takes a plural verb: A number of files are missing. The number of takes a singular verb: The number of files is missing from the log.
Examples
Use The number of complaints rose.
Use The amount of waste rose.
Avoid The amount of people in the hall rose.