This is all off the beaten path, so I suspect the answer is no one knows. Font tables have a limit of 65k characters, but this ceiling can be busted in whacky ways using multiple lookups, useExtension... Practically, font building tools / operations crash (mysteriously), stalls (mysteriously), or slows to a crawl (indistinguishable from stalling), and the Cantonese Font about pushes the limit.