We never got a number, but Suder spoke very authoritatively about killing people in the past in the Maquis. They were even creeped out about how violent he was. Including Torres and her ragefits as the status quo.
But maybe keeping him in his quarters was a fit of brilliance. You never can tell when having a killer around will come in handy. Like when you somehow let a group of protoKlingons take over your ship.