I solved my problem, but without using a helper function. here is how I solved it:
$(window).on('action:topic.loaded', function (event, data) {
var replynum = data.postcount - 1;
console.log(replynum)
});

I have tested it and it works, every time a topic is loaded I can get the postcount.
However, I am still unable to know why my helper function is not being executed :/

I registered a hook, I wanto get data from the post (the data I want is post count) but somehow my function is not being fired. am I using the wrong filter ?

"hook": "filter:post.getPostData", "method": "replynum"

Thank you !

is it possible to make simple calculation inside the templates?
for example, I want the number of the replies only (not the number of posts) which means:
number of posts -1.

@baris it works! thanks a lot!

Is there a way to render the category of a post in a topic page ? something like this work inside recent page :
<a href="{config.relative_path}/category/{topics.category.slug}"> {topics.category.name}</a>
is there anything similiar where I can add inside post.tpl ?
am trying to avoide using js for this.

I am trying to add a sorting mechanism to the recent page (forum/recent) exactly like the one the category page (/forum/category/) where the topics can be sorted either from newest to oldest, oldest to newest, most posts. is that possible?