Cleveland Police confirmed 165 men have now claimed they were assaulted by staff at HMP Kirklevington, near Yarm.
The claims of bullying and excessive physical punishment date from the 1960s to the 1980s when they were teenagers at the facility, when it operated as a mixed remand centre.
The force said no arrests had been made and the investigation was ongoing.
A Ministry of Justice spokesperson said: "Anyone with evidence of abuse should bring it to the attention of the appropriate authorities so it can be examined as a matter of serious concern."
Kirklevington was converted to a resettlement prison for adult male offenders in 1992.
Allegations of historical abuse have been made by former inmates of a remand centre on Teesside.