Hey Peregian, I don't believe there is a limit to the number of devices that can be added to a user account in the Home app. The bigger issue is the demands and capability of your wifi network, distance to the Chromecast, obstructions, signal strength etc. If you have hundreds of units and 600 students pulling from one wifi network/router and the signal is coming from the basement, I'd imagine there would be problems with streaming etc.
The second question which you didn't ask but is relevant and logically follows your explanation of intended use is: Providing access to one user account isn't very secure if those "multi-dwelling units for students etc." will need to log in to a single/central Google account for access. Doing so opens up a pandora's box for credit card access, ownership of limited use licenses for purchases and rentals of digital content as well as "distribution" of copyrighted content made from that single Google account. If each student or resident will have their own separate SSID/password and by extension, their own Chromecast either outright or provided for each room, then each student should have their own handheld device with Google Home installed and use their own Google account to pay for and access content.
