Show count in view grouping
Using this answer as a base you can add views_field_view to include a view of content filtered by term.

In more details 

1. Create a view of terms that includes count of content using each term


Create a view that displays terms
enable aggregation
add relationship Content with term
add sort content uuid with aggregation type: count (optional)
add field content uuid with aggregation type: count
add field taxonomy term


Your view should look similar to the following

 

2. Create a view that will display the content


Include contextual filter on the vocabulary of your interest (tags in this case)
Add any filters or fields that you need 


This view will look similar to 



3. Add last view as a field to first view (step 1.)


Download and enable views_field_view module
Go to view from step 1. and 


add field Taxonomy term: Term ID
add field Global view ) (made available from views_field_view)
on the settings find the view from step 2.
use {{ raw_fields.tid }} as contextual filter



Next screenshot shows the changes from original view and settings for view field



You are done

Your view now should display the terms with count and all the content that have used it.