As far as i remember, that is a retired number that is no longer used. However, I do know that the best player to ever use that number was Roger Staubach, a legend of his time.
The Dallas Cowboys do not officially retire jerseys, but I don't think any one would have the testicular fortitude to wear number 12 for the Cowboys.
Edit: Here are some stats for ya:
Statistics: In 11 seasons with Dallas, Staubach threw for 22,700 yards and 153 touchdowns. At the time of Staubach's retirement in 1979, only Otto Graham had a higher career passer rating than Staubach.
Accolades: 6 Pro Bowls, 5-time All-Pro, Ring of Honor, Hall of Fame.
Longevity: After missing five seasons due to commitments to the Navy, Staubach joined the Cowboys in 1969. He spent two seasons as Craig Morton's backup before taking over the starting job midway through the 1971 season. He remained the starter for the next nine seasons.
Intangibles: Staubach proved he was a winner even as his supporting cast changed throughout the decade of the 1970s. When Dallas won its second Super Bowl title of the decade in 1977, only two starters from the 1971 squad remained- Staubach and tackle Ralph Neely. Few quarterbacks have been so successful while leading a team through a transition period as Staubach during that decade.
There is not currently a #12 on the Dallas Cowboys because the Dallas Cowboys retired #12, which was the number of former player, Hall of Famer, and Ring of Honor member Roger Staubach. Technically the Dallas Cowboys do not retire jersey numbers, so the above assertion that #12 is a retired number is false. However, that is just a technicality because nobody would ever have the guts to wear that number since Roger Staubach is a living legend.
13
13-3
The most wins the Dallas Cowboys had in one season was 13. They did this twice, finishing 13-3 in both the 1992 and 2007 seasons.
The Dallas Cowboys' first year in the National Football League was 1960. In 2014, they will be competing in their 55th season.
The Cowboys 2007 regular season record was 13-3.
The Cowboys 2007 regular season record was 13-3.
The Cowboys have never gone undefeated. The best team records were 12-2 in 1977, 13-3 in 1992, and 13-3 in 2007.
Brett Favre has faced the Dallas Cowboys 13 times in his career, including regular-season and postseason games. In those meetings, the Cowboys have been victorious nine times.
Baltimore Colts 16 - Dallas Cowboys 13.
Dallas Cowboys 30 ---Buffalo Bills 13
NFL Monday Night Football - 1970 Pittsburgh Steelers vs- Dallas Cowboys 13-1 was released on: USA: 13 September 1982
NFL Monday Night Football - 1970 Houston Oilers vs- Dallas Cowboys 13-6 was released on: USA: 13 December 1982