So Oda would have to handle 12 SHs ? It was already complicated with only 9 of them.
Its only "complicated" because he has to deal with 50 other arc related characters. As we near the climax of the series, the number of characters he introduces will have to decrease and instead he will have to focus on the ones he's already introduced.
Remember, Wano has been built up since Punk Hazard. If 12 strawhats existed during FI, it wouldn't have been an issue. But then Punk Hazard brought in Law, Caesar, Kin'emon and Momo. Then, Oda sacrificed half the strawhats in Dressrosa to introduce, literally 30 more characters (SHF and Villains) and focus on them. Then he switched the focus on respective strawhats and focused on the gargantuan amount of BMP characters along with Bege. Not only that, but nearly ALL of those characters (plus those from Zou) are now in Wano along with Kaido's side, plus 8 supernova, plus 9 scabbards.
Its a lot, but if Oda can juggle 60+ "main" characters this arc, having 12 or 13 crew members in the end really isn't that big of a deal. Having Jinbe, Carrot and Yamato join this arc, IMO, really don't affect how much panel substance Oda provides the main characters because he's already been limiting their presence with the sheer number of characters he already has. Once he starts to lower this, having 12/13 crew members isn't really a problem.