In a formula column, the NB_ELEMENTS() function displays the number of elements contained in selected columns.
I counted the number of items in the Document Received column to find out how many.
NB_ELEMENTS(Received documents, Subscription)
I counted the number of items in the Documents received column + the number of items in the Subscriptions column to find out the number of items in my client's file.
In the same way, I can count the number of items linked to another table.
I counted the number of invoice statements in my Invoice column issued to a given customer.