Posts for a run
The tally gives the number of posts. It does not choose their length.
Corner and post length
This tally is one straight run. For an L shape, count each side on its own. Give the corner post to one side only, or the two tallies will both claim it.
A panel 1.8 m high is often set on a 2.4 m post, with about 600 mm in the ground. A heavy gate or soft ground may want more in the hole. Check that against the post you buy.