I think 2 looks clean on both 14 or 18 count, but any higher than 18 I'd go to one. I actually just experimented with different thread weights on my challenge project.
On the Prince, outline of the cube, and blue circles I used 3 strands (thinking on the black 14 count, it wound need to be denser to not show black), Portal button and cube I used 2 strands, and on the Rainbow Road background I used 1 strand to try to make it look more see-through. You can kind of see the difference; overall 2 I think is standard.