IRS website says
You received a payment, but it wasn’t the full amount of the Economic Impact Payment. The maximum credit is $1,200, or $2,400 if married filing jointly, plus $500 for each qualifying child.
Then:
When you file a 2020 Form 1040 or 1040SR you may be eligible for the Recovery Rebate Credit. Save your IRS letter – Notice 1444 Your Economic Impact Payment – with your 2020 tax records. You’ll need the amount of the payment in the letter when you file in 2021.