The point is that you get the worm test done before you worm. They will tell you whether your horse has any worms and needs doing. I have two horses. I had a test done this year and both were negative, they did not recommend I worm them. I will send another sample off in the summer and if there is a result, I will worm them, if not, I won't bother.
I would add that I keep my two horses at home on clean fields that had been used for hay for years.
It depends what you wormed your horse with last as to when you do the worm count. You do the worm count at the point at which the next wormer would be due. i.e. Equest is due every thirteen weeks so you do worm count that many weeks after you last wormed
Do a worm count a couple of days before your next worming is due, which will be the number of weeks as advised on your wormer. Personally, much prefer Abbey Diagnostics (and they are cheaper)