There are many ways to achieve this. However, I just follow your convention.
Here is the code that you can follow-
$discussPosts = DB::table('post')
->join('postConversation', 'post.id', '=', 'postConversation.PostId') //you used Did, I am not sure which one is a foreign key, but it seems that PostId is a foreign key after checking your table structure. Adjust your foreign key if it is not working.
->select('post.*', DB::raw('count(postConversation.*) as postConversationCount'))
->where('post.mentorid', Auth::user()->id)
->where('post.menteeid', $userid)
->where('postConversation.seen', '!=', 1)
->get();
dd($discussion) //Result