See-More: C-Cause... I-it sounds cooler?
Sometimes, an organization, group or gang needs specific rules. Like how many members it can have. Its fairly routine with evil organizations, though it may appear with heroic groups as well.
This is a case when the number of the group is defined and, apparently, immutable. There are times when a justification is given during the course of the story, but other times there is no explanation whatsoever, except that it sounds better.
Please: When adding examples in the justified category, do state the explanation given in series.
- In the first anime adaptation of Fullmetal Alchemist, the Big Bad employs a maximum of seven homunculi at any one time, giving them codenames based on the Seven Deadly Sins. This is in contrast to the original manga, where the homunculi are literal embodiments of the Seven Deadly Sins which split off from the Big Bad and thus cannot gain new members (though they can be "rebooted" to some extent).
- One of the biggest examples can be said to be Fruits Basket: The cursed members of the Sohma family are always 14, due to being possessed by the animal spirits of the Zodiac, plus the spirits of the Cat and the God of the legend.
- Of all schools shown in the Harry Potter series, Hogwarts is the only one whose students are divided into four Houses, created by the four founders.
- In The Elenium, the Styric Pantheon is aptly known as The Thousand, or, literally, "ten times ten times ten", which they picked because it was an auspiciously round number. One god who'd lost a finger had wanted it to be the "nine times nine times nine", but more deities than that had already come into existence.
- In the Dreamblood Duology, the Prince of Gujaareh has an Exotic Extended Marriage of 256 wives: the goddess Hananja is associated with the number four, so 256 (four times four times four times four) is especially holy, as is the Prince as Hananja's Avatar.
- Organization XIII of Kingdom Hearts series is a group of the few elite Nobodies who still look human (Nobodies being what's left of a particularly strong-willed person who's lost their heart to darkness, their body and soul continuing to operate as emotionless husks) and are willing to do anything it takes to get their hearts back to be real people again. They are the main villains of Kingdom Hearts: Chain of Memories and Kingdom Hearts II, and the Villain Protagonists of Kingdom Hearts: 358/2 Days (well, three of their members are anyway). Several games after the group's debut, it's revealed in Kingdom Hearts 3D: Dream Drop Distance that Xehanort's true purpose in creating the Organization was to get empty vessels he could implant with copies of his own heart for use as candidates for the 13 Seekers of Darkness he needed as part of his larger scheme to create a legendary weapon. In reality, the group only has 13 members for a very short period of time. Starting with just six members, it eventually grows to 12 over the course of many years. Finally, Roxas becomes the last, Number XIII... but then Days reveals that Number XIV joins a few days after him. Days later reveals that Xion was not considered a real member of the group, she was basically a clone of Roxas intended to replace him as the Organization's Keyblade-wielder who'd create Kingdom Hearts for them in case he didn't work out. In Kingdom Hearts III, the group's second incarnation, filled by the successful 13 Seekers and dubbed "the Real Organization XIII", actually has a total of 15 members, with one brought in to serve as the thirteenth active member even with two other members already on standby as "reserves". Hilariously, the new Number XIII was Xion. Guess she does qualify as a Real member.
- A large number of governmental bodies always have exactly X number of members (e.g.: the U.S. Supreme Court always has nine members, unless there's a vacancy). There are far too many of these to list them all, but it's safe to say that for any given such body, the reason that it is always exactly X number is usually "It's convenient."
- Historically, a jury (in common-law countries, at any rate) always had twelve members (with the exception of Scotland, where the number was 15), with the historical justifications ranging from the numerological to the lazy. Modern juries are an aversion: many jurisdictions call for smaller juries in criminal trials, and even the most conservative (like the American federal court system, which insists on the old-fashioned twelve-person jury for criminal cases) allow for the parties to agree on a smaller jury in civil cases.
- D.Gray-Man gives us the Clan of Noah. It is made very clear that there are only 13 members; one of the reasons for the existence of the Fourteen is a mystery until it is revealed that he is actually part of the Millenium Earl, their leader. No reason was given yet to why they have limited members.
- Blake's 7 is only a borderline example because the titular Rag Tag Band Of Misfits are never actually called such in-universe, but for some reason they never spent more than a couple of episodes down to six or fewer main characters before picking up someone new. This persisted even after Blake himself was Put on a Bus.