There is no Apple's IP address with Private Relay. Apple is using 3rd part companies as its exit nodes to avoid this "whitelist apple ip addresses" concern.
Private Relay guarantees that users can't use the system to pretend to be from a different region, so you can continue to enforce region-based access restrictions. Details about the proxy IP addresses will be available as an article associated with this session.
Though I haven't been able to find the aforementioned article.
[1]: https://developer.apple.com/videos/play/wwdc2021/10096/
https://developer.apple.com/support/prepare-your-network-for...
And their IPs, as mentioned in that article:
https://mask-api.icloud.com/egress-ip-ranges.csv
Seems to be mostly Fastly IP addresses right now, but I'm sure that'll change over time.