Here is 1 parent collection and 4 children collections.
While the parent collection is expanded, the user can clearly get the items unber each collections.
While "Show items unber subcollections" is selected.
https://s3.amazonaws.com/zotero.org/images/forums/u6241286/529iitnzkn26qq9zsml1.png
While "Show items unber subcollections" is not selected.
https://s3.amazonaws.com/zotero.org/images/forums/u6241286/qw6k2y7p2n0x2y9vqbz3.png

how about show the item number like this?
As an example：
while the parent toggle to be collapsed, show the whole number of items unber the collection and subcollection
https://s3.amazonaws.com/zotero.org/images/forums/u6241286/jc3bac2lxqg2vst8n8kx.png
while the parent toggle to be expanded, show the own number of items unber each collection and subcollection
https://s3.amazonaws.com/zotero.org/images/forums/u6241286/qw6k2y7p2n0x2y9vqbz3.png