It was a number that easily relates to the decided upon game cap (120), which makes six characters 5% of the game. It's easier to work with percentages that are whole when it comes to caps like this, and 5% is a decent number for one large cast, especially when the next round number (in relation to 120) is 12, which is large for a cast and 10% of the game.

Six characters is also room for a reasonable amount of characters from any given typical canon -- there are some exceptions, like how huge Homestuck is, but your average show, book, anime, game will have a cast of main characters that fits around this number as well. It's essentially not so small that most canons would be filled up easily, but also not large enough to monopolize a lot of room. Hopefully that makes sense!