Hi @dhineshkumar 

@Rajesh Chopade1  has given a solution to your requirement using an On-Change client script. If you still prefer to use reference qualifier, you can take logic from the Script Include provided by him to get the count. Please note that the Script Include will not be Client Callable in that case so be aware of the syntax. This Script Include you can call within your reference qualifier to get the number of reportees under a manager.

Thanks and Regards

Amit Verma