Is there circuit breakers with 12000 ampere rating, and what about the max ratings available

I'm not sure of the application in question... I've worked with 18KV OCBs on really big feeders, and 2.5 MW 25KV main breakers on Cat 3516s. In my experience, breakers get as big as wanted... it depends on the cost...

For low voltage application (less than 1000 Volt), maximum current rating is 6300A.
For medium voltage application (1000 volt - 52000 volt), normally up to 5000A. Some manufacturer provide higher current rating, up to 10.000 A for special application.
For high voltage application, normally up to 3150A.
Normally, the higher the voltage, the lower the current rating will be.

What application is the CB for?