Now that Android 8.1 Oreo's out, we don't know what's next for Nexus 6P users outside of monthly security updates.
The company hasn't confirmed Android 8.1.1 or Android 8.2 so there's no guarantee those updates arrive in 2018 ahead of Android P.
At this point, we'd expect the next release to be the August security update. Google's security updates typically land on the first Monday of the month so keep an eye on carrier Android update schedules for specifics in the weeks ahead.
As for Android 9.0 P, it's set to arrive in Q3 after a stint in beta for Pixel, Pixel XL, Pixel 2, and Pixel 2 XL users.
We expect developers to release Android P for the Nexus 6P (in ROM form) so keep your eyes peeled for a stable release.