Like so many things, it depends on the circumstances, the light, the subject and what is required to get the shot etc.
Here is one shot at ISO22800 as I needed a relatively high shutter speed as the Platypus was moving:
Z9 + 800 f6.3 PF, 1/500s f/6.3 at 800.0mm iso22800
Here is an Osprey that was moving fast and I was at 1/4000sec and thus required ISO5000.
Z9 + 400 f2.8E FL VR, 1/4000s f/4.5 at 560.0mm iso5000
This Forest Kingfisher was in very low light but at least was sitting still. However it still required ISO7200!
Z9 + 500 f5.6 PF, 1/125s f/5.6 at 500.0mm iso7200
Here is one shot at ISO22800 as I needed a relatively high shutter speed as the Platypus was moving:
Z9 + 800 f6.3 PF, 1/500s f/6.3 at 800.0mm iso22800
Here is an Osprey that was moving fast and I was at 1/4000sec and thus required ISO5000.
Z9 + 400 f2.8E FL VR, 1/4000s f/4.5 at 560.0mm iso5000
This Forest Kingfisher was in very low light but at least was sitting still. However it still required ISO7200!
Z9 + 500 f5.6 PF, 1/125s f/5.6 at 500.0mm iso7200