Safire 2nd generation video door entry systems allow the connection of up to 9 external boards (camera VIMOD module) with up to 500 master monitors.
You can then add 5 slave monitors to each master monitor. So, per apartment, there can be up to 6 monitors.
In total: 500 master monitors + 2500 slave monitors = 3000 monitors
And each VIMOD module can be connected by RS485 to up to 8 "accessory" modules (such as keyboard, button panel, etc...), forming an installation of up to 9x9=81 modules in total.