I really need to know exactly what you want to achieve.
It may be that you have to query it if the reply_count value isn't available in that template hook. Without knowing what template hook, it's going to be difficult.
Also, template hooks are officially deprecated so you should be using the built in Template Modification system for templates; though, again, if the parameter isn't available to the template then maybe the template hook is more appropriate.
Unfortunately I can only speculate at this point.