Which is the Arc Number in the series?

But the series has:

• 3 main realms
• 3 aspects of being
• The Twilight Town trio, The Sora Recovery Trio, TT's Disciplinary Committee, Disney Castle's trio, 3 Original Masters Trio
• 3 types of keyblades

Plus, what makes the other numbers not too general?

 

Note that most of those triplets didn't exist in the first game. They were all shoehorned into the second. None of this resonates with the themes of Kingdom Hearts: friendship, strength of hearts, etc. The three realms? One has only been seen once, and basically only existed to provide a pretty backdrop for the final fight of KH2. The trios of characters? Roxas was originally in a group of four with Hayner, Pence, and Olette - He wasn't part of a trio until Days. The Disciplinary Committee and Disney Castle trios existed before Kingdom Hearts, so they clearly weren't created to enforce the theme. And the Masters have never been thematically connected; Yen Sid has nothing to do with the other two, and there's far more connection between Eraqus and Xehanort as a duo. The three types of Keyblades? Another creation of later games; none of that was codified until late KH2 into the hand-helds.

For something to be an arc number, it has to have some specific relevance to the plot; seeing it should instantly remind you of something important, or throw up a red flag. In that sense, the number 13 is closer, but even then it's sort of retconned into importance rather than important from the start. It's just a number of people. A less common number of people, granted, but still no more.

1 is a pretty general number, since everything is made up of ones; as for why the others are less so, it's mostly 'cause they're bigger. It's less common for a group of seven or thirteen friends to become close, and when it comes to mystic artifacts you don't as often find that many. When those numbers do occur, it's worth more attention. But like I said, even with that in mind the numbers are kind of shoved into the series.

 

1. The three keyblades were in fact in the first game. Sora's Kingdom Key (Realm of Light), Mickey's Kingdom Key D (Realm of Darkness), and Riku's Keyblade of People's Hearts.
2. What do you mean Yensid has no connection with the other Masters? Eraqus stated that he was a close friend. In BBS Yen sid is the one who notifies Aqua that Eraqus is dead and in Recoded he is the one who tells Mickey that Xehanort has been revived. It is also kind of implied that those three used to a trio of students under the same master. Plus, as a side note, all of their names are anagrams that are kind of important(Square,Disney, and No Heart/Another).
3. You do realize The World That Never Was, Traverse Town, Twilight Town/Yen Sid's Tower, and Castle Oblivion are also part of the Realm Between?
4. True, Roxas was a part of the Twilight Town group, but it wasn't real...Plus, Days also proved that Hayner, Pence, and Olette were a trio even before Roxas joined his Days trio.

So 7 is not a pretty general number?