If you find your Latest Posts from your profile, you can count how many of those have been posted on one particular thread. It's a much easier and quicker way of checking it, rather than skimming through the entire thread itself.
I don't think there's any function atm that actively counts how many times you post in each thread.
There is another way. If you go to Forums and find the section of the thread, find the thread, then scroll over the views/replies and it will say "You have X number of post in this thread, last X date"
When you look at the index of threads in any forum at NB, you will see that the number of replies in that thread is also displayed in front of the thread title.
Click on this and a pop up box will open which will let you know which users posted in it and how many times, including yourself if any. You can even check the posts they made from there- just again click on the number of post that user made in that box. You do not have to manually count the posts or locate them.