As of my knowledge cutoff in September 2021, multiple supermax prisons exist in the United States. However, the number can change as new facilities are built or existing ones are repurposed or closed. It’s important to note that supermax prisons are highly secure facilities designed to house inmates who are considered the most dangerous, violent, or high-profile within the correctional system.

Some well-known supermax prisons in the United States include:

1. ADX Florence – Located in Florence, Colorado, ADX Florence is often considered the most secure and restrictive federal supermax prison in the United States.
2. United States Penitentiary, Administrative Maximum Facility (USP Florence ADMAX) – Also located in Florence, Colorado, USP Florence ADMAX is another federal supermax prison known for its high level of security.
3. Pelican Bay State Prison – Located in Crescent City, California, Pelican Bay State Prison is a state-level supermax facility operated by the California Department of Corrections and Rehabilitation.
4. United States Penitentiary, Marion – Located in Marion, Illinois, USP Marion was formerly a supermax prison but was downgraded to a high-security facility.

It’s worth mentioning that other correctional facilities across the country may operate with high-security protocols or house inmates deemed as maximum security, but not all of them are officially designated as supermax prisons.

For the most up-to-date and comprehensive information on the number and locations of supermax prisons in the United States, it is recommended to refer to official sources such as the Federal Bureau of Prisons or individual state Department of Corrections websites.