My familiarity with Sharepoint and Flow is limited (Read. I looked into them today)
So i have some learning to do, but to paraphrase
Its entirely possible to have 300 folders, a single flow that would find any new files created among those 300 folders, go to a sharepoint list (not sure what this is yet, but imagine its similar to an excel table) reference the parent folder of the file, and find any email addresses associated with that folder in the list, and send an email to them indicating a file has been uploaded and is available. I'm assuming it would have to track what files have been processed in order to avoid multiple notifications.
off-topic i know, but could i have is send an email to anyone that has shared access to the folder? At least i wouldn't have to maintain an additiona list on sharepoint with this method.. i think
Sounds super complicated, but i understand the logistics of not having/maintain 300+ flows