Count links to the same table

[F_T]

Hi,
I have a “tasks to do” table, which includes

1. a linked field “prerequisite tasks” that links to the same table
2. a date field “completed date”
3. a formula field which counts 1 if the task is completed and 0 if it is not.

I want to count, for each task, the number of unfinished prerequisite tasks. However I can’t seem to create a count field as Airtable insists on using a link field to ANOTHER table.
Did I miss something?