Consider the suffix -ry or -ery,* which comes to us from Latin (-arius) via Old French (-er, -ier) via Middle English (-erie) and, attached to a noun or verb, can mean either a craft, study, or practice (husbandry, midwifery), a collective plural (Jewry, nunnery), or a place in which a particular activity takes place (bakery, hatchery).
So witchery can mean:
Witches collectively, and
One of my favorite lines from the Charge of the Goddess has always been: For behold, I am Queen of all Witcheries. Apparently there are multiple witcheries, and She's queen of them all. Andrew Mann said of Her in 1597: She has a grip of all the Craft. That's quite a claim.