As a good rule of thumb, higher numbers usually mean the most recent version. This website is nice for that, you can see all the trainers as they were developed and released, going from like +5 to +16 as time goes on, and maybe the game expands and adds more features. (Civ5 adding a Religion currency later, as an example)
Not always the case though. Sometimes the options might end up not working or cause problems and get removed later...but still.
Like Kalas said, +5 would mean the trainer has 5 options to use (a trainer with Infinite Health, No Reload, Infinite Ammo, Medpacks, Grenades would be a +5, for example)
I don't know if they include the "Activate Trainer" option in the + numbers though.