Okay, posting my own answer because I actually found new sources/info! So as Paulie_D mentioned, Seven's full designation (which you can also find on the Wikipedia page for her character) is "Seven of Nine, Tertiary Adjunct of Unimatrix 01." If you go from beginning to end of the designation, you're going from most specific to most general description of her placement within the Borg collective.
In this case, "7 of 9" means that she's the seventh drone in her particular group/unit of 9 drones within the ship she's on, based on info from the fandom Wiki page on Borg drones. (And group sizes can vary — in The Next Generation episode "I, Borg," the drone nicknamed "Hugh" is "third of five" within his unit.) "Tertiary adjunct" tells you Seven's particular group's place within their unimatrix, which Memory Alpha defines as a "subdivision ... used to designate groups of Borg drones and regions of space."
Based on the information that we get from Voyager about Unimatrix 01, unimatrixes (unimatrices?) are the broadest/largest subdivisions of drones within the collective. They then get broken down into "adjuncts" and possibly even into smaller units within those adjuncts, hence how you get designations like "Seven of Nine" and "Third of Five" rather than "Seven of X billion and counting."