It's very hard to do. Streaming definitely isn't it because groups can have different ratios of casual listeners, and depending on the demographics of their fanbase their streams will also vary, and people from different countries behave differently, in some countries they might prefer buying songs and album to streaming.
The second best metric is concert attendance, but this one also comes with its own set of problems, for one you have to factor in the GDP of each country because let's say a group has 100k fans in the US and they also have 100k fans in a poor country, just based on how much money they have you will get significantly less people able to buy a ticket even though it's the same number of hardcore fans.
On top of that, even in the case of two countries where people make about the same amount of money you'll have a difference in culture when it comes to concert attendance, in some places people are more willing to attend concerts of artists they don't know or they only know them on a casual basis. There is also ticket pricing.
The biggest problem with using concert attendance as a metric is the existence of a predetermined number of tickets, as much as labels and event organizers try their best it's hard to judge how many people a group can get to attend their concert, and they will try to be conservative with their estimates, so if a group is constantly growing then their current attendance numbers are gonna be way off relative to how much they can actually pull.
In my personal opinion the best metric is album sales, BUT you still have to factor in a lot of variables, doing a 1 to 1 comparison of album sales between two groups is gonna be incredibly inaccurate.
1) How many version does the album have
2) Price of the album
3) Length of time between comebacks
4) Where the sales are coming from: The same as concert attendance, whether it's a poor country or a rich one will matter a lot, how big is the physical market is in that country, and the strength of the fandom culture in that region.
This also relates to the second variable(price) because not only do albums have different prices from each other, but they can heavily vary depending on where they are sold, an album that cost $20 dollars in Korea might be sold for $50 in another country, And because of that groups who have more fans in areas where albums are more expensive will have their sales reduced.
This is just a guess, I'm gonna use BTS and give my estimate to how big the fandom of other groups is relative to theirs(hardcore fans, no casuals):
- Blackpink 1/4
- NCT 1/4
- Seventeen 1/5
- TWICE 1/12
- TXT 1/14
- Stray Kids 1/16