Bullet work question

[Monarchos1]

Anybody know what the minimum number of workers needed for a work to be classified as a bullet? Looking at Derby PPs, I see Musket Man had the fastest of 3 at CD on April 18, but it isn't designated as a bullet. I can understand if its 1/1 like Friesan Fire's at KEE on April 19, but is there some specific number of horses that must work at a track/distance for the fastest of those to be designated with a bullet?