When the ERC was introduced in the Coronavirus Aid, Relief, and Economic Security Act (CARES Act of 2020), it allowed for a maximum amount of $5,000 per full-time worker, if eligible employers pay their employees during government-mandated shutdowns.
Companies with 100 or fewer average full-time employees could claim the full amount for all employees while those with more than 100 employers could only claim wages paid to employees not providing services. Since then it's been revised three more times to the point where the IRS created a 2020 vs 2021 ERC comparison chart.
The ERC was first revised in the Taxpayer Certainty and Disaster Tax Relief Act of 2021 (TCDTRA) which increased the maximum allowable amount in 2021 to $7,000 per employee, per calendar quarter. Amendments were also made to allow tax-exempt organizations and businesses that received PPP loans to claim the credit. The definition of a large company was also changed from 100 or more employees (2020 claims) to 500 or more (2021 claims).
In the American Rescue Plan Act of 2021, a Recovery Startup Business that had gross annual revenues of less than $1m in the last 3 years is now allowed to claim up to $50,000 per quarter in Q3 and Q4 of 2021. The employer tax offset was also changed from the employer's portion of Social Security tax (2020 claims) to the employer's portion of Medicare taxes (2021 claims).
Finally, during the Infrastructure Investment and Jobs Act of 2021 (IIJA), ERC eligibility was restricted in Q4 of 2021 to Recovery Startup Businesses only. Eligibility for severely financially distressed employers was also removed for ERC claims in the fourth calendar quarter of 2021.