Is there any good source of how many converted Portuguese Jews (cristãos-novos) kept on practicing Judaism in secret during the time of Inquisiton?

I read a book by a Brazilian autor who says that most forcibly-converted Jews in Portugal, which were then called cristãos-novos, kept on practising Judaism, though in secret. He also affirms that at the eve of the abolition of the distinction between purely Christian families and families of Jewish origin (more than 2 centuries after they were forcibly converted), a solid majority were still practising it, and few were sincere Christians. That seemed to be a hard to believ assumption, since their practice of Judaism would have been totally oral and there was not a single case of reverted Jews back then. Does anyone have any source on the subject?