Yeah but I don't think we need to report the highest value found. It seems Windows gets away with 1024, but to play it safe I've picked the lowest on contemporaneous hardware, which is still higher than what Windows uses since 20 years.
I wonder if the value Windows reports has any correlation to D3DDEVCAPS_DRAWPRIMITIVES2EX or D3DDEVCAPS_DRAWPRIMITIVES2. It might be that the 1024 comes from a runtime provided emulation of old draws based on the newer driver interfaces. There's a test in test_enum_devices_caps_callback ("HAL Device hal device caps does not have D3DDEVCAPS_DRAWPRIMITIVES2EX set") - does it fail on HW that reports something bigger than 1024?
It is also possible that this value only has a meaning for execute buffers, i.e. D3DEXECUTEDATA::dwVertexCount.