Taxpayers who received an Economic Impact Payment, should keep Notice 1444, Your Economic Impact Payment, with their 2020 tax records. They may be eligible to claim the Recovery Rebate Credit on their tax year 2020 federal income tax return if:

they didn’t receive an Economic Impact Payment, or
their Economic Impact Payment was less than $1,200 ($2,400 if married filing jointly for 2019 or 2018), plus $500 for each qualifying child they had in 2020.
If a taxpayer didn’t receive the full amount of the Economic Impact Payment for which they were eligible, they may be able to claim the Recovery Rebate Credit when they file in 2021. Individuals do not need to complete information about the Recovery Rebate Credit on tax year 2020 Form 1040 or 1040-SR when filing in 2021, unless eligible to claim an additional credit amount.