Short answer: 403.4 out of a possible 1,261.6.
Long answer: Here are some stats for FBS teams since 1979 (when passer efficiency was first used), categorized by # of attempted passes.
--(Min. 12-24 atts.) 403.4---Tim Clifford, Indiana vs. Colorado, Sept. 26, 1980 (14 attempts, 11 completions, 0 interceptions, 345 yards, 5 TD passes)
--(Min. 25-49 atts.) 317.4---Bruce Gradkowski, Toledo vs. Buffalo, Nov. 1, 2003 (25 attempts, 23 completions, 0 interceptions, 435 yards, 6 TD passes)
--(Min. 50 atts.) 199.2---Chris Redman, Louisville vs. East Carolina, Nov. 14, 1998 (56 attempts, 44 completions, 1 interception, 592 yards, 6 TD passes)
Joe Namath
Colt McCoy
tom brady
Brett Favre has thrown the most touchdown passes in NFL history, with 508.
Colt McCoy
Colt has thrown the most touchdown passes in a college career
Tim Couch
Matthew Stafford (Detroit Lions) 2010
Marcus Dupree
he has thrown for six touchdowns on two occasions
Daryle Lamonica of the 1969 Oakland Raiders - six touchdown passes in the first half. Did not even play the 2nd half because the Raiders were winning 42-0 at half time. Lamonica could of had 12 touchdown passes if he had played the entire game.
Tim Tebow(461) Tebow didn't have 461 touchdown passes!