Something like twice as many rookies sign deals with NFL teams as college free agents relative to the number of those who are actually chosen with a selection in the NFL Draft, and this has been true since the league expanded offseason rosters form 80 to 90 in 2011.
The Pittsburgh Steelers are a somewhat good example, though have been better in others. They had just six draft picks, but signed 10 players as college free agents. Other teams have signed dozens of players as college free agents. It really just depends on how much you filled out your roster prior to the draft, and the Steelers did their due diligence picking through the XFL.