Class posts, posts in the library and at the Quidditch pitch are generally counted. When you make a post in any one of those places it counts as one (this works for anyone, whether they're a student, professor, or completely random character playing an NPC for a plot). OOC posts aren't, otherwise someone could go up from first to seventh year in a day just by spamming. Sometimes we have special plot boards where posts are counted there as well, but we don't have any of those just now.
When you say "how/when does it change" I assume you mean at what number does a character go from first to second year? The chart for that can be found
here.
As for posts in the FPP, people generally claim those in order to add to their character's post count to help them to move up a year. This is particularly useful with 7th year characters who, as you can see need to gain a higher number of posts in order to graduate.
Hope that makes sense!
[carys]~ Ca
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