If you are asking about the EIC (Earned Income Credit), your income also has to be within certain ranges. If you are asking about a different credit, please post in the comments below.
The EIC (Earned Income Credit) is a range based on the amount of earned income you have, your filing status, and the number of qualifying children you have. If you earn too little or too much, the amount you receive for the credit is scaled based on that amount. Most taxpayers who qualify for the credit will not receive the full amount.
You can read more about the income ranges for your filing status and number of qualifying children below.
Also see this for income ranges and amounts of the credit: 2016 EIC Table
What are the qualifications for the Earned Income Credit (EIC or EITC)?