According to Live Science, there is an estimated 2,000 "at large serial killers". This was determined by Thomas Hargrove, a retired investigative journalist. He asked some of his contacts from the FBI to calculate the amount of unsolved murders that were linked to at least one other murder through their DNA database, because the FBI defines a serial killer as someone who murders two or more victims with a length of time between crimes for "cooling off".

After searching their database there was only 2% that met the criteria Hargrove asked about. In a quote with Live Science Hargrove says that there are 220,000 unsolved murders since 1980, and that at least 2,000 are unrecognized series of homicides.

As someone who watches and listens to multiple true crime shows this is very interesting but super creepy. Even though this is just an estimate, the idea that there is even one active serial killer doesn't rest easy with me.