The number in "Subscribed" shows how many of the threads where you posted have been answered. Usually you see the unread thread titles written in bold font. That's how you can differentiate them. Read/open them, and you'll notice that then number will disappear when you've viewed them all.
Worst case, visit the site in Safari instead of using the iPF app. Choose the "Settings" link near(ish) the top right to see all your unread messages and threads.
Unlike the iPF app's Subscribed tab, this only shows the unread stuff.
Normally the bold threads in the iPF app are fine, but now and then you'll miss one and it ends up several pages down the list; which can be a pain to track down. That's when it's nice to visit the website and do a little house cleaning.