Here are some options that are playful or descriptive, but not necessarily official:
* A clack of typists (playing on the sound of typewriters)
* A flurry of typists (emphasizing their fast pace)
* A legion of typists (referencing a large group)
* A battery of typists (imagining them working in unison)
Ultimately, the best collective noun for a group of typists depends on the context and the desired tone.