When dealing with a trigger or multiple triggers, it will always default to 0 because it is listening to the system to see who qualifies for the campaign. Once someone qualifies, they will be put into the flow steps immediately. It won't show you the number who qualify because the system goes right to work when someone has qualified.
If you were to use filters, then you would see a number because the smart list would find everyone who fits the criteria to qualify for the campaign before the campaign is scheduled.