Hmm, the Japanese article does say 20 characters originally. That much Im confident about. (For any other Japanese translators out there, for the record, the original line is 「オリジナルキャラクターの善と玲を加えた20名から最大5人を選出して、プレイヤーの好きな編成で探索やバトルに臨むことができる。」
But youre right that the math might be a bit weird. Heres the screenshot they provided of the character select screen and what originally led me to state what I did in tandem with the information explicitly stated in the article:
[character select screen]
I didnt notice this until you pointed the numbering discrepancy, but I think the article might be doing a weird semantics thing that allows us to both potentially be right. If you look closely at that screenshot, you might notice that while the Persona 4 protagonist is a selectable character, the Persona 3 one isnt, despite being a part of the party lineup up top. The only reasonable thing we can conclude from this is that it means the latter is the character that the player chose to play as at the beginning of the game.
If we take that to be the case and assume that the reverse would happen if you were to select the Persona 4 protagonist, then the articles contents can still be technically correct: you can still choose from 20 characters, excluding the character you picked to play as in the beginning because he has to be a constant fixture in the party by default and cant be removed. The original Japanese article doesnt outright state that, but theres nothing in there that outright denies it, either. These sorts of articles are usually just written based on whatever PR the developer has handed out to the publications, so the number itself is at least correct; its just how that number is interpreted, of all things, thats potentially messy.
You could be fully right, too, Im not trying to deny that. Bear in mind that the Velvet Room attendant stuff that I wrote was always speculation on my part and hopefully that was made clear in the original post. I made that assumption based on the number presented in the article and that screenshot up above and all Ive done is demonstrate that theres possibly a way still for the remaining character slots to be four instead of three depending on how people choose to calculate it. Its weird, but issues like this arent unheard of with Japanese game reporting, so I suppose well find out one way or another soon enough by the time the game is out in Japan!
(Edit: I know theres been a lot of hubbub about being able to potentially form parties that dont have either protagonist about them, but unless I missed an article or something somewhere, I havent seen any information anywhere that explicitly states as much. That lack of official confirmation that I know of is ultimately what makes or breaks my math in the end. Just wanted to address that point specifically.)