A dozen at the bare minimum I guess.
Red Velvet, MAMAMOO, aespa, Purple Kiss, Itzy, TWICE, BLACKPINK, SNSD, Dreamcatcher, Brave Girls, Kep1er, BTS, SHINee, TVXQ!, GFriend (sometimes I mistake Yerin/Eunha), Oh My Girl (Jiho by exclusion), BIGBANG (Seungri by exclusion)
Over 30 groups
bts twice bp txt itzy aespa mamamoo shinee
rv sometimes i get confused, ateez, skz i think i get confused, exo some chinese members i think because i couldnot recognize some people in mama mv, gidle sometimes some members idk, i can actually name like 10-15 nct members so i'm proud of it
there's no way i only know 8 grps fully i'mm probably forgetting some grps 
You gotta name the groups for an honest and accurate answer
bts twice bp txt itzy aespa mamamoo shinee
rv sometimes i get confused, ateez, skz i think i get confused, exo some chinese members i think because i couldnot recognize some people in mama mv, gidle sometimes some members idk, i can actually name like 10-15 nct members so i'm proud of it
there's no way i only know 8 grps fully i'mm probably forgetting some grps
If you ever need/want help telling Stray Kids apart sometimes, I'd be happy to help. There's also this video:
How to tell stray kids apart for locals - YouTube
And I'm sure there's other videos like it for other groups. Haven't checked, but there probably is. Okay, never mind, I just checked and there is. Feel free to use such videos if you ever need/want them in the future. Have a great day/night!
Too many 
Let's see here... Off the top of my head approx 45ish
Wow you can really tell I'm mostly a casual listener for ggs huh 
urm actually a lot haha
seventeen, bts, exo, got7, ikon, winner, wanna one, x1, enhypen, txt, treasure, shinee, nu'est, gfriend, blackpink, twice, red velvet, rocket punch, aespa, iz'one, ive, mamamoo, kep1er, ioi, stayc, itzy, brave girls, (g)i-dle, everglow, oh my girl
nct maybe like 7 of them i can match the name and faces correctly...
BTS, BP, Twice, Red Velvet, Dreamcatcher, TXT, Aespa and EXO (sorta, not all the Chinese members, but the core now and Lay). Not all of NCT but NCT 127 I think I can.
EDIT: I'm looking at other people's list and I forgot these groups: BigBang, Itzy, Gfriend and Mamamoo
EDIT 2: I could probably do almost all of SVT, but I'm not as confident
EDIT 3: Ok this is like my third edit, but I forgot G-IDLE.
So went to wikipedia and did a quick count and I know all the members in 49-57 group. I'm confident in 49 of them and there are eight groups I know I used to know all the members in but haven't payed much attention to lately and have lowkey forgotten. Like there's 1-2 members in them that I'm not fully confident I would recognize outside of a group setting anymore.
Know all the members:
Give me a minute to refresh my memory:
Okay someone above challenged me/doubted me saying it doesn't count unless I can name them so even tho it's 4:55 in the morning I'm doing this *rolls up sleeves* 
2. SNSD: Yoona, Taeyeon, Tiffany, Seohyun, Yuri, Sooyoung, Sunny, Hyoyeon (ex: Jessica)
3. BTS: Jungkook, Jimin, V, RM, Suga, J-Hope, Jin
4. CLC: Yeeun, Seungyeon, Sorn, Eunbin (Eunbean lol), Yujin, Seunghee (ex: Elkie Chong)
5. Infinite: L, Sungkyu, Woohyun, Sungjong, Sungyeol, Dongwoo (ex: Hoya)
6. B.A.P: Bang Yong Guk, Zelo, Daehyun, Youngjae, Jongup, that pos Himchan
7. BLACKPINK: Lisa, Jennie, Rosé, Jisoo
8. Aespa: Karina, Winter, Ning Ning, Giselle
9. Dal Shabet: Woohee, Subin, Jiyul, Kaeun/Gaeun, Serri, Ahyoung (ex: Vicki)
10. EXO: Kai, D.O, Lay, Baekhyun, Sehun, Suho, Chen, Xiumin, Chanyeol (ex: Tao, Kris, Luhan)
11. Big Bang: GD, TOP, Taeyang, Daesung (ex: literal human garbage Seungri)
12. DB5K: Jaejoong, Xiah Junsu, Micky Yoochun, Max Changmin, U-Know (Yunho)
13. Super Junior: Donghae, Eunhyuk, Siwon, Kangin, Sungmin, Yesung, Kyuhyun, Ryeowook, Heechul, Shitdong Shindong, Leeteuk (ex: Han Geng, Kibum (?)
14. f(x): Amber, Luna, Krystal, Victoria (deceased: RIP Sulli)
15. SHINee: Taemin, Onew, Minho, Key (deceased: RIP Jonghyun)
16. TWICE: Nayeon, Momo, Sana, Chaeyoung, Mina, Tzuyu, Jihyo, Jungyeon, Dahyun
17. SISTAR: Hyorin, Bora, Soyu, Dasom
18. Kara: Seungyeon, Nicole, Jiyoung, Gyuri (deceased: RIP Goo Hara)
19. Miss A: Jia, Min, Fei, Suzy
20. Girl's Day: Sojin, Yura, Minah, Hyeri
21. EXID: LE, Hani, Hyerin, Junghwa, Solji (ex: U-Ji/Yuji & someone else)
22. Hello Venus: Lime, Yoo Ara, Nara, Yoonjo, Alice, Yooyoung
23. 4Minute: HyunA, Gayoon, Jiyoon, Sohyun, Jihyun
24. ITZY: Yeji, Ryujin, Yuna, Chaeryeong, Lia
25. Block B: Zico, Taeil, P.O, U-Kwon, Kyung (only unknown: Jaehyo)
26. Wonder Girls: Yenny, Yubin, Sohee, Ahyoung (?), their leader HA:TFELT (only remember her solo name)
27. U-KISS: Kevin Woo, Soohyun, Eli, AJ, Hoon, Jun (only one I can't name by face)
28. GOT7: Yugyeom, Jackson, Mark, Jinyoung, Youngjae, BamBam, JayB (/JB/Jaebeom)
29. TXT: Beomgyu, Yeonjun, Taehyun, Soobin, Hyuningkai
30. MAMAMOO: Hwasa, Solar, Moonbyul, Wheein
....and more
w00t, I DID IT!
So basically most important 2nd Gen groups. I started giving up on knowing all the groups' members names after that and can only name all the members of a few 4th Gen groups cuz it's honestly not worth trying to learn all their names honestly 🤷🏻♀️
dozen or more...better with GG thn BG I guess
From the top of my head :
big boy groups : TVXQ, BTS, Big Bang, Exo, Monsta X, Btob, Winner, iKon, Stray Kids, TXT, Super Junior, Astro
big girl groups : Blackpink, Twice, Mamamoo, SNSD, Wonder Girls, 2NE1, Miss A, Sistar, f(x), Itzy, aespa
Mixed : KARD
my faves : IZ*ONE, Loona, Weeekly, WJSN, woo!ah!, (G)I-DLE, Everglow, fro9,
probably missing other groups
I'm pretty good with names. Let's see...
NCT, Astro, GOT7, Monsta X, Seventeen, Blackpink, aespa, Twice, Red Velvet, Mamamoo, Stray Kids, Oneus, IVE, BigBang, Super Junior, Shinee, TVXQ, EXO, BTS, TXT, Enhypen, itzy, g-idle, Ateez, A.C.E, Kard, AB6IX
There's another handful where I know most of the members, but would struggle to recall of them like Billlie, Victon, VIXX, 2PM, SNSD, Day6, Winner, iKon, Oh My Girl, SF9, The Boyz, CIX
edit to add AB6IX because I do know them all now that they're 4 members!
Too many to list lol it’d probably reach like 3 quarters of a hundred groups 
many lol im even lazy to list
Back then I need the month to learn the name from the SNSD members.....now I remember drom group I like in a second 
Most known 3rd and 4th gen groups and some 2nd gen ones.
Quite a lot I think; I'm pretty good with names.
GOT7, The Boyz, Itzy, Twice, Blackpink, BTS, Monsta X, Red Velvet, Everglow, Mamamoo, Seventeen, Pentagon, NCT, 2NE1, SNSD, Girl's Day, Miss A, Wonder Girls, 2PM, BTOB, Shinee, EXO, Stray Kids, Ateez, Gfriend, Super Junior, IKON, Astro, Aespa, Golden Child, Big Bang, Kard, Seventeen, Day6, Gidle
I know all the members of my faves and a few members from Twice, Weeekly, StayC and Purple Kiss but still get confused on some 
1. BTS
2. Red Velvet
3. Blackpink
4. Mamamoo
5. Girls' Generation
6. aespa
7. Itzy
8. SHINee
9. BIGBANG
10. Twice
11. Brown Eyed Girls
12. Secret
13. Sistar
14. Wonder Girls
15. Girls' Day
16. G-IDLE
17. Exid
18. AOA
19. TXT
20. f(X)
21. Miss A
22. EXO
23. Loona
24. 2NE1
25. Kara
27. T-ara
28. IOI
29. Got7
30. 4minute
31. Dalshabet
32. Ladies' Code
Twice
So 10!
Doesn't surprise me too much since I can also name most regulars of the top 12/13 european football teams 
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