Yes, but in this implementation, that post counter is likely maintained in your profile, and is retrieved in real time along with your name, avatar, etc. So if you've made 100 posts, every message shows 100 posts. Once you make another post, every message shows 101 posts until you make another.
In other systems, one message would say 100, meaning this was the 100th post (and this stays with the message forever), and the next post is labeled 101, etc. So, you can see the chronology of the posting...
There's no right or wrong way to do this... just different styles. One is a running message count, one is a count total.