What do you mean by "number the posts"?
Each post has an ID, which is available in its URL. Example:
http://n2.nabble.com/Differences-between-Nabble1-and-Nabble2-tp3925029.html(ID is 3925029). Is the ID useful for your case?
If you just want to just mark some posts (add them to your favorites), you can use the star feature.