How to get concurent running instances using WCF

WCF throttling behavior gives us the facility to manage the max concurrent instances, what I want to do is control the instances created by the client were wcf services are consumed. for e.g:- if client calls a method hosted on wcf service and the execution is going to take long hours I want to restrict the calls upto 5 instances. My question is can we control the max instances manually where in if 6th call is made it should return the exception ot client saying exceeded the max limit, although this can be done by using collections but is there any other way (using wcf calsses) were we can handle these scenario.

Thanks in advance
Sandeep