When the so called experts say there are 12 million Catholics in China they are first estimating the number for the official state controlled church which is around 6 million. Then they are estimating that the secret, true church is about the same size as the state controlled church. So 6 million in the state controlled church and 6 million in the secret church makes 12 million.
Devout Catholics who lived in China and worshiped in China told me that the secret, true church is much larger than the state controlled church. They also said many true Catholics pretend to be members of the state controlled church for appearance and personal safety but are actually members of the secret true church.