WARNING, NUMBERFAGGING IN THIS POST
I finally found some EN pre-debut numbers that shouldn't be too inaccurate. Pic related was taken about 3-4 hours before Mori's debut (first row).
This comment on /hlg/ corroborates Gura's sub count to be in that range
https://warosu.org/jp/thread/S26182778#p26182954I got the pic from Ame's debut thread, posted 27 minutes after it finished
https://warosu.org/jp/thread/S26229260#p26244247