Since the restructuring of the Entertainment Analysis and Development divisions into the Entertainment Planning and Development divisions, the group numbers and team structures have become less rigid.
These are as close as we can get to solid teams and the games/IP they focus on. That said, one group may help another group. They are much more flexible now.
EPD Group 1 - Assists 2nd part and 3rd party developers.
EPD Group 2 - Assists 2nd part and 3rd party developers. Mario spin offs, Smash Bros, Kirby, Yoshi, etc..
EPD Group 3 - Zelda.
EPD Group 4 - 1-2 Switch, Labo, Ring Fit Adventure
EPD Group 5 - Splatoon, Animal Crossing (With Group 9).
EPD Group 6 - Assists 2nd part and 3rd party developers.
EPD Group 7 - Assists 2nd part and 3rd party developers.
EPD Group 8 - Mario.
EPD Group 9 - Mario Kart, ARMS, Pikmin, Animal Crossing (with Group 5).
EPD Group 10 - Pikmin, Super Mario Maker, New Super Mario World,.
Not to mention:
- Intelligent Systems: Rumored to be working on a Paper Mario game and didn’t have a lot of staff working on FE: 3H (majority was from Koei Tecmo)
- Monolith Soft: just increased their staff to 243 people and 4 studios. They’re likely delegating one studio to help with BoTW2 and the others to work on Xenoblade Chronicles: Definitive Edition and their upcoming new IP (or new Xeno game).
- Next Level Games: Just released Luigi’s Mansion 3. They’ll likely have another project in the pipeline.
- Retro Studios: While they haven’t done much since DKC: TF, they have assisted in developing MK7 and are working on MP4.
Nintendo can delegate different studios to work on various projects. Not to mention, they have often collaborated with third party partners with their big games (I.e., Astral Chain, Mario + Rabbids, FE: 3H, TMS: #FE, Hyrule/Fire Emblem Warriors, etc.).
So I think it’s not like they have some studios just twirling their thumbs. They just take time. Nintendo has published about close to a dozen games a year (even more before with 3DS/Wii U). I don’t think even Sony or Microsoft have published that many a year, let alone develop and release, unless someone can tell me otherwise.