I’ve had my Z9 for two years now. Shooting professionally full time for the past 18mos. The only time I’ve had to worry about battery capacity was when shooting video. We chew through batteries on a video day. For stills, it’s rare for me to use a full battery. Even more rare to pop in a second battery on a stills-only shoot. Usually it means a full day.
Just in case, I also have a PD in the car to recharge an exhausted battery. A road trip in the fall was the first time I even thought about battery capacity, but mostly because I wasn’t consistently charging at night.
Obviously, YMMV. Getting into more wildlife and bigger trips, I could consider a third battery for my Z9. Probably, I’d look at better options to charge what I have though. Unless I start to hit that worrying “2nd battery close to exhaustion” state.
Depending on what, where, and how I’m shooting the spare either goes in a pocket on my person, my bag (loose), or a small pocket in my binocular chest harness.