I put this as a comment, I may as well put it down here as an answer with a bit of expansion.
There is no canonical term for a random grouping of Witchers.
The one term we have heard used of 'groups' of Witchers is a School. But the School is a term directly tied to the location, tradition, and methods of their training. Geralt is of the Wolf-School, and we've heard of or met Witchers from the Cat, Griffon, Bear, Manticore, and Viper schools.
However, as I said, this is a specific term for a training tradition of Witchers, not a generic term for an ad-hoc group of them.
While this is not specifically supported by the books, there is a very good reason, in-universe, as to why there is no such term. You don't see Witchers in groups. When Witchers are on The Path, they are solitary. They run into one another on occasion, work together sometimes, but eventually split off and go their separate ways. While on The Path, the largest number of Witchers we have ever seen traveling together is 3...and this was temporary, not something intended to last throughout the entire season they were out and about. The only time they really gather up is during the Winter, when they return to their School to wait out the snows.
To give a real-life example, there is no term for a group of Honey Badgers, because they are generally solitary creatures...thus there is no reason to give a name to something which does not exist. So, like the Honey Badger (in more ways than just this), there is no term for a group of Witchers.