Is it possible to install two or more cards?

Yes, you can install two or more cards. This will double the performance under macOS in some apps and allow to connect even more displays.

Never mix NVIDIA with AMD cards, because the system might become unbootable.

We recommend using cards with the same type or series and memory size.

NVIDIA cards requiring NVIDIA Web Driver e.g. GTX900/1000 series can only work as a single card, because installing Web Driver disables the second card.

Older NVIDIA cards from GT600/GTX600/700 series and Quadro K5000 will work in a multi-GPU configurations with a default macOS driver.

Due to the power limitations, only cards with a single 6-pin PCIe power connector or no power connector can be used in a multi-GPU configurations.

After installing two or more cards they will be visible in Graphics/Displays section in System profiler.
Running a simple benchmark like LuxMark will show the benefits of a multi-GPU configuration.   You can download it here: https://wiki.luxcorerender.org/LuxMark_v3