For tax year 2021, the AMT exemption is $73,600 (Unmarried Individuals [other than Surviving Spouses]), $114,600 (Joint Returns or Surviving Spouses), and $57,300 (Married Individuals Filing Separate Returns). This is the amount that gets deducted from your AMTI (alternative minimum tax income) before applying the AMT rate.
The exemption amount starts to phase out (is reduced) at AMTI levels of $523,600 ($1,047,200 if filing jointly).