Hi @jan_mac ,
I done a test on my side, you can refer to my steps.
Here is my library called library1 with two folders:

Inside folder Logistics, there are four subfolders(folder sub1 has one file, folder sub2 has 2 files, folder sub3 has 3 files and folder sub4 has 4 files):

In my app, I have a Gallery that displays all subfolders inside folder Logistics:
Filter(library1,IsFolder = true &&'Folder path'="library1/Logistics/")

In this gallery, beside each subfloder, there is a label to show the file count of each subfolder:
CountRows(Filter(library1,'Folder path'="library1/Logistics" &"/"& ThisItem.Name &"/"))

Best regards,
Allen