You got me thinking
I couldn't find the most runners left in scoring position(has to be out there somewhere) in a season, but found that the 1976 Cincinnati Reds left the most when they stranded 1328 runners on base during that season. Sidenote - The 1976 Reds won the world series that season.
Well, obviously stranding the most runners is a stat that needs context. They also probably had the most runners on base, and scored the most runs. Too lazy to look it up so I'll call that a mindfact.