Troubleshooting guide
How to fix the problem
Connect the monitor with a suitable cable or adapter and select the advanced mode and display layout in System Settings > Displays. Check the number of screens supported, resolution, and conditions with the lid open or closed according to your exact Mac model. The number of USB-C ports alone does not tell you how many independent displays can be connected.
If the problem persists
Check the exact Mac model and the supported number of screens, resolutions and lid open mode. Different models have different limits; a simple hub doesn't automatically remove them.
Request a repair