multiple homekit hubs for thread network devices

That depends on your HomeKit accessories, and where they are, and what kind of connectivity they use. Not all accessories have the same capabilities (full vs. minimal Thread accessories).

Only one home hub and thread border router is strictly needed for functionality. Thread accessories are supposed to form an interconnected mesh where some have the functionality to send along signals from others Thread devices (Thread router).

Node roles and types - OpenThread

Multiple Apple TV boxes (or HomePod speakers) can each be a HomeKit home hub, but with one active hub per home, and the others on standby. They can all work together to make the Thread network more robust.

If most or all your accessories are minimal Thread devices, then the Thread network may need some extra devices with Thread routing capabilities (Thread accessories or dedicated border routers).