256 players were drafted in the 2018 NFL Draft over a three-day period. Some of those players will never play one down in a regular season NFL game. On the other side of the coin, many college players that went undrafted could end up having long careers in the NFL.
Kurt Warner went undrafted in the 1994 NFL Draft. He was a training camp invitee with the Green Bay Packers. He was released before the season and then found himself in the Arena Football League. He was successful there and then signed by the St. Louis Rams. Starting QB Trent Green suffered an ACL injury in the preseason and Warner was named the starter. He went on the win the Super Bowl that season. Warner is now a member of the Pro Football Hall of Fame.
Other recent undrafted players that had success in the NFL: RB Thomas Rawls, LB Zach Orr, WR Willie Snead, CB Malcolm Butler, RB Corey Clement, WR Robbie Anderson and many others.