At last year's I/O, Google surprised many of us by announcing that the Android P beta would not only be available on the Pixel line-up, but also on select devices from other manufacturers. Project Treble played a huge role in making that possible, and things are going to be even better this year: more companies will be part of the Q beta than P.
Speaking to the Android Developers Backstage podcast in episode 110, Googler Iliyan Malchev from Project Treble says that "the number [of companies participating in the beta] is bigger for the upcoming Android release." You can listen to the exact moment here (warning: Pocket Casts link). He doesn't mention an exact number, but there should be more than the seven from last year — Essential, Nokia, OnePlus, Oppo, Sony, Vivo, and Xiaomi.
It will be a couple of months before we know who the newcomers are, since the new Android release hits beta with DP2 and is historically announced at I/O. But if you ask me, logic says there should be a few more Android One makers there (BQ, GM, maybe even LG and Moto). We could also be looking at Redmi and Poco if they count as separate from Xiaomi now. Whether Samsung, Huawei, or even non-Android One LG devices are included is much more questionable though. Still, these are exciting times, and it sounds like beta testing the new version of Android will be less and less reliant on having a phone straight from Google.