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.