Seven pirates licensed by the government they are supposed to oppose — a numbered set whose number is a legal quota rather than a ranking.
Filed by its count, not its title
Reduced as a phrase, “The Seven Warlords of the Sea” would sit on The Ledger — but that would be reducing wording this reference chose. The set is filed under 7 · The Inquiry instead, because 7 is what its author chose, and a chosen number outranks an imposed one.
Seven pirates granted immunity in exchange for nominal service to the World Government. Membership is a contract, not an achievement: seats are vacated, filled, and revoked, and the story eventually abolishes the system entirely.
This is the rarer kind of numbered set — a quota. Because the number is administrative, every vacancy is a plot device, and the reader is trained to notice an empty chair. Compare the Four Emperors, where a missing member would be a catastrophe rather than a vacancy.
Seven is also, quietly, the least contentious number a fixed roster can take. Large enough that no reader expects to meet all of them promptly; small enough that a reader still believes the author intends to.
Oda has not explained the choice, and this reference does not invent motives for authors. What can be said is that seven arrives in Japanese popular fiction pre-loaded — the Seven Lucky Gods, the seven-member sentai team, Seven Samurai — as the number of an assembled band whose members are individually distinct. Turning that warm convention over to a set of licensed criminals is a joke that only works because the convention is so settled.