Count fields show the number of linked cards of a specific type that meet a condition.
For example: show the number of task cards linked to a project card that are not yet completed.
This field can help you get quick insights by just looking at top-level cards that group other cards.
Start by opening the card type.
Next add a field and select the "Count" type.
Now you can select which related card you want to consider.
Finally you can set the conditions when the related card should be counted.