But the magic number could also be n+p-q where 'n' equals the number of watches you already have, 'p' the number of watches you still desire and 'q' the number of watches you possess but don't want to wear anymore...
Another way to find this number is to consider that you need watches which go with evening dress, others for corporate wear, others for casual wear, others for sports gears... In each situation, you need white, black, blue... dial watches (in different materials) to match with the color of your dress (for instance, I love platinum and white gold watches with blue dials and straps to match with blue suits and shoes) and have the choice between 3, 4, 5, 10... watches of different sizes/shapes/complications for each situation (depending on your mood of the day)...
At the end, I found that the magic number could be not very far from a few hundreds!
And that number could have been only a start if my wife didn't come and said 'stop'...