Pwnz0rz Man
Smash Lord
Mario's first portrait is named 001 in the common5.pac of 3.0. I think if they wanted to give certain characters more costumes than others, they could cut into the previous evens. I'm not 100% sure of this, but I think it would work like this.The limit (at least with cBliss, I can't speak for ACE though I imagine it's similar) comes from the way the cosmetics are named, specifically, the battle portraits. Mario's first portrait is numbered 000, his next one is 001, all the way up to 009 (or, like, 005 or 006 in vBrawl). Donkey Kong's starts at 010, so if they gave Mario an eleventh costume, it would overlap with Donkey Kong's battle portrait. If you gave Donkey Kong another costume, it would intersect with the next character, who starts at 020 (Link, I think.)
Wario is special, because he had more allotted battle portraits from the beginning since he had 12 costumes anyway, but every other character follows the naming pattern above. PMBR could, theoretically, change it, but I doubt they would bother renaming and writing code to repoint at 460+ files.
For instance. MenSelchrFaceB.000-010 could be designated for Mario while DK could keep his current range of 011-017.
Yoshi in theory could start at 040 instead of 041 in order to obtain a Brown costume, but it would theoretically mean Samus couldn't have 10 outfits/colors.