Seven of Nine is one of only a handful of the Borg species that has we know of by a name. One is another, a name given to himself. Picard as an assimilated Borg was named Locutus. By whom the name was chosen is, as far as I know, not stated.
But we must can assume that individualistic names is far from being a Borg thing. So does Seven of Nine mean that she is a seven of nine in a group, orderly and like a set? Or are the numbers nominal and the ‘of’ has no real meaning? Considering how many Borgs there are (assimilated) Single digit names seem hardly enough to cover all individuals in the Borg Collective. Likely there are thousands of individuals so there could be “1432 of 9” or “2 of 34254”.
Likely too is that there is no hierarchy. For example, I do not think Six of Nine out ranks Seven of Nine because of a one-digit difference. It would seem illogical to have a hierarchy in a collective of unranked drones. There is something socialistic or communistic about the Borg.
Likely, a short double-barrelled single-digit name is easier than “23482 of 967” as a name. As in real life uncommon names can either make it very easy or difficult to remember.
Another question is, do all individuals have unique names or do some have the same name like “David” or “John” in the English speaking world.
And is Seven her first name, and Nine her last?
So many questions so few answers.