I'm not sure if this could reliably be done. I mean, yeah, it can, but that would require telemetry to track how many devices are actively using the OS. They could limit the privacy impact by not storing device-specific info on their end, but the information still gets processed (it has to be, otherwise they can't differentiate between different devices to increment the counter.)
Something like this isn't likely to happen because the target audience for GOS is generally very anti-telemetry, and adding something like this would be pretty controversial and only give more ammunition to those people who regularly do everything they can to criticize and discredit GrapheneOS.