Most states have a WARN act that covers even more. For example, California - https://edd.ca.gov/en/jobs_and_training/Layoff_Services_WARN...
While federal law has:
Plant closings involving 50 or more employees during a 30-day period.
Plant closure affecting any amount of employees. Layoff of 50 or more employees within a 30-day period regardless of % of workforce. Relocation of at least 100 miles affecting any amount of employees. Relocation of a call center to a foreign country regardless of the percentage of workforce affected.