Constant STREAM_COUNT_UNKNOWN 

pub const STREAM_COUNT_UNKNOWN: u64 = u64::MAX /* 18_446_744_073_709_551_615u64 */;

The value super::PublishDone::stream_count carries when the publisher cannot state an exact count.

Section 9.9: “If the publisher is unable to set Stream Count to the exact number of streams opened for the subscription, it MUST set Stream Count to 2^64 - 1.” The sentinel is indistinguishable from an exact count of 2^64 - 1.