Connecting up to 40 clients is not possible at the moment. Our current spacedesk Driver for Windows 10 only supports up to 16 display connection.
Mirroring the main desktop screen to multiple displays is possible, just that Windows Display Control Panel/Settings user interface does not support it. This applies to spacedesk and all other displays.
We are considering to add this option but we are currently very busy with higher prioritized features and bug fixes.
We can only suggest the following:
1. You can use any third-party software that can mirror the main screen to all monitors connected (e.g. Actual Multiple Monitors)
2. OR You can try this trick with your Windows 10 primary machine and spacedesk.
-- connect all spacedesk clients and make sure that all displays are in extension mode first
-- open spacedesk Server window, click the menu button "..." then select "Clear All Display Settings"
-- press "Windows key + P" combination, then select "Duplicate"
Please keep in mind that connecting to more than 3 displays can affect the performance of each viewers connected (e.g. screen update lag may be observed) and make sure you have a powerful primary machine (latest display graphics adapter) that can handle many displays.