Dave and Rachel (TAR20) performed the fewest tasks in completing one leg in Paraguay: 0 (unless you count going down a slide as a task in which case they would be on par with every team that has ever taken a fast forward)
*Disclaimer: I haven't verified this* but I propose that Nicky and Kim (current season: 23) performed the most tasks in completing one leg: UAE2 = 6
1. coin riddle
2. dune buggies
3. & 4. both sides of the detour : dress camel, prepare food
5. wave pool speed bump
6. rapids roadblock
For this I classify one complete leg as being from pit start to pit stop (even if it is fake, virtual, whatever). I think the only contention would come from the double leg in season 6; in that case they are the F/F team that has completed the most tasks in one leg or the team in the current racing era, i.e. since the introduction of the u-turn i guess...
I think it's a pretty neat record, contradict me if you can!