Audio Objects are currently sent on a first-come, first-serve basis. Those that don't make it are mixed into the main mix before being sent to the sink. Also, you may unexpectedly hit the limit sooner than expected if you try to send multi-channel audio objects, use multi-positions, or use Spatial Audio like diffraction where each image source becomes an audio object.
That said, with Wwise 2021.1.8 and PS4/PS5, priority is also sent as metadata to help determine what should be played as audio objects and what's sent to the main mix.
More info in Audio Devices