Since the harbinger names were inspired by Commedia dell'arte, the first thing that came to my mind after reading the post is, what would no.10's name be, if they do exist?

I found that the only name from Commedia dell'arte that is not yet used by harbingers is Innamorati, which means "The Lovers". And as the name indicates, it refers to 2 persons instead of 1. Also, Innamorati are the only type of character that don't wear masks.

Therefore, if there is an empty no.10, it's probably meant to be empty. If wearing/not wearing masks means being/not being known as a harbinger, then Innamorati can be the harbinger whose indentity is not known by other harbingers. In other words, nobody else except themselves and the Tsaritsa know that they are Innamorati.

I think that there has always been 12 seats for the director and harbingers, and people were assigned to these seats by the Tsaritsa. But Childe said the Jester was the first harbinger", right? Isn't the first harbinger "the Captain"? Well, it could be that Pierro aka the Jester was once the first and only harbinger. After some time, the Tsaritsa decided to recruit more harbingers, so she created the list of harbingers, promoted Pierro and told him to find more "talented" people to fill the list.

However, the Tsaritsa has the final decision on the recruitment. So Pierro (or other harbingers) might actually recruited no.10 Innamorati but he never knew they were approved by the Tsaritsa.