It's very important that you are able to identify a witch, if you ever run into one (which you very well may). That's why we're going to include Grandmamma's tips on how to spot a witch:
Now that we know how to recognize a witch, let's talk about the witches our narrator comes across.
The witches are all kind of less intense versions of the Grand High Witch. They, too, are mean, but just in a less specific way. We learn at the beginning that they are always "plotting and scheming and churning and burning and whizzing and phizzing with murderous bloodthirsty thoughts" (1.7), but we don't really get to see much of this at the annual meeting.
There are a few witches who speak up – one gets smoked, one gets reprimanded, and one is responsible for sniffing out our narrator – but mostly they're followers. They're quite good at repeating everything the Grand High Witch says, and they sure are skilled in the art of flattery. Can you imagine saying to someone, "'Whoopee! […] You are brilliant, O Your Grandness! You are fantabulous!'" (8.16). Try that one out on your teacher and see what kind of reaction you get.
So why do we need all the witches when they don't really have individual personalities? Well, the story wouldn't be quite as exciting if our narrator just turned one witch into a mouse. Eighty-four is much more satisfying.