To count the number of linked records in a field, you can create another field using the "Count" field type. This will automatically provide the number of records that have been linked to a particular record in a designated field. (For a basic overview of the count field type, see "Guide to Formula, Lookup, Count, and Rollup Fields.")
Example: Counting Assigned Tasks
Let's suppose that you have a table of team members, and a table of tasks that need to be completed. Using a linked record field, we can link employees to the tasks to which they've been assigned.
Once you've done this, you'd like to be able to see, at a glance, the number of tasks each employee has been assigned. To do this, create a new field with the count field type (you can title this new field "Tasks Assigned"). You will then be prompted to select name of the field on this table that holds the linked records you would like to count. In this case, the title of that field is "Tasks."
The new count type field you've created will automatically count the number of linked records in the designated field. If you add or remove linked records from the designated field, the count field will automatically update to reflect the new number of records.