Racing posts made simple.
If you have two posts, the horse will run relays between the two.
If you have four posts, the horse will run in a circular manner between the four.
For instance, if you want a race track, you need four, one at each corner.
However, if you're going to do something like barrel racing, you would only need two with some fun obstacles like hay bales in the path between the two.
You can see how I did my race track with jumps below:
I'm doing a new ranch at the moment to do the Ark Builder LTW. This one will be much more 'western' in flavour rather than 'english'. I'll post a picture of the barrel racing training area after I'm done.