Using a single domain to serve multiple clients globally using CloudFront with anycast IP addresses.

Looking at the launch blog post:

You must add these IP addresses into any relevant allow lists.

So yes, add all 21 IP addresses.

Note that if you're using the apex record of your domain and you're using Route 53 then that number is reduced to three. Ref: https://aws.amazon.com/about-aws/whats-new/2025/04/amazon-cloudfront-anycast-static-ips-apex-domains/

As an aside: While using CloudFront Functions to route to regional endpoints works and is one way to do it; consider using regional/location-based DNS instead. Lower cost and less "moving parts".