Why Fans Think The Seven Deadly Sins Doesn't Make Sense As A Group
The debut episode of the hit anime "Seven Deadly Sins" introduces the Seven Deadly Sins, its titular group of heroes, as a band of disgraced traitors. However, Elizabeth, the princess of the kingdom of Liones, knows that the Sins are not actually traitorous but were framed with the explicit purpose of necessitating their exile. So, she sets out to find and reform the group so that together they can retake Liones from the Holy Knights, who control the kingdom in a sort of military rule.
Later on, the series reveals in flashback that the formation of the Seven Deadly Sins began upon the onetime king of Liones witnessing a supernatural vision of a group of seven powerful warriors. According to his vision, that group would be capable of defending his kingdom from the Ten Commandments, a group of ten demons aiming to take over the world for demonkind. With the help of the series' powerful protagonist Meliodas, himself one of the Sins, the king was able to assemble the team he envisioned.
However, the Holy Knights of Liones — an army comprised of many of the kingdom's strongest warriors — engineered a scenario that would make the Sins appear to be traitors to the naive public, hence the need for their exile lest they be imprisoned or killed. It's through the lack of the Sins' protection that the Holy Knights were able to take over Liones.
Because "Seven Deadly Sins" is a trope-y anime series, the powerful members of the Sins prove capable of handling the threats of first the Holy Knights and later the Ten Commandments. That said, some viewers have nevertheless noticed that the group's formation doesn't hold up to scrutiny.
The Seven Deadly Sins aren't a great match for the Ten Commandments
u/Wise-Tourist shared their gripes about the formation of the Seven Deadly Sins in a thread posted to the series' subreddit titled "why make a team of 7 to fight a team of 10?" Their issue is essentially contained in its entirety in that title: Because the Seven Deadly Sins were formed explicitly to combat the Ten Commandments, they find it confusing that the Sins might not, at the very least, match the group they were formed explicitly to defeat in number.
Of course, the most straightforward answer to the question is that "Seven Deadly Sins" author Nakaba Suzuki wanted to utilize two Biblical terms without having to modify their associated numbers. That said, an ensuing discussion in the thread's comments section attempts to suss out why the Sins team's numbers might make sense in-fiction.
u/Ryuuji_Gremory first suggested that the members of the Sins are the only non-evil warriors powerful enough to contend with the Commandments, given that their non-Sins allies are considerably weaker, and thus wouldn't make ideal candidates for an expanded roster. u/Wise-Tourist replied that, though the Sins prove powerful, the King of Liones had no way of knowing he chose the seven strongest warriors prior to their Seven Deadly Sins training, and thus could have applied the same regimen to a greater number of team members.
In response, u/Ryuuji_Gremory then recalled the king's vision containing some level of specificity about who to recruit, providing some rationale for his choices at the very least. That said, neither user was able to fully explain why, in the series' fiction, a team of seven would be suited to fight a team of ten.