The asn1 crate provides three builtin Choice enums itself - asn1::{Choice1, Choice2, Choice3} - which support 1, 2 and 3 choices respectively. I assume that is what lilyball is referring to. But as you correctly point out the custom derive supports mapping enums with more variants to CHOICE just fine, so the builtin enums are not a relevant limitation.
Edit: Here's an example of a CHOICE implemented with rust-asn1 that has more than three variants[3].
[1]: https://cryptography.io/en/latest/x509/reference/
[2]: https://cryptography.io/en/latest/x509/verification/
[3]: https://github.com/pyca/cryptography/blob/be6c53dd03172dde6a...