I proposed a table because in your image (second table and your queries attempts with group/flatten) it seemed that you want control all the other information. 
But if only a list with the total you can use a LIST not a TABLE
Cellui-ci:
```dataview
LIST length(rows.file.link)
FROM #task AND -"Templates"
FLATTEN client
GROUP BY client
```
Ou celui-là:
```dataview
LIST WITHOUT ID client + " | " + length(rows.file.link)
FROM #task AND -"Templates"
FLATTEN client
GROUP BY client
```