A lookup count column is a special type of computed field that computes the number of items in the target list that point to the current item.
Let’s say I have two lists Departments and Employee Data. Now in Department list I want to add new column which will calculate how many employees are there in that department.
Here’s how we need to setup our lists:
1) Create a Department list. And add your department values in a title column.
2) Create an Employee Data list. In “Employee Data” list I renamed ‘Title’ column to ‘Employee Name’. And then add column ‘Department’ which will be lookup column pointing to the title column in the Department list.
3) After adding some test values in “Employee Data” list the list will look like this
4) Now go to “Department” list, add a column of type "Lookup" referring to the “Employee Data” list and you'll notice that in the drop-down area where you define the lookup, you'll have a new option called "Count Related". This is here automatically because it recognizes that the “Department” list has a lookup pointing back to this one. Select that Count Related option.
5) Now your Department list will have a column counting how many employees are associated with that Department.