How many impee can be programmed simultaneously?

I have a project planned to go to production soon, in production we want to shorten the programming time as much as possible, so I want to ask how many impee can be programmed simultaneously in the same WiFi network?

Depends on your bandwidth. The factory process tends to only be loading tens of kB to test/bless an imp, and it can be done optically to many devices concurrently.

Given a good router, likely hundreds.