I don't exactly remember but more than half or a high number? Has a minimum too, if people can't publicly be witches then you got no info and need a scheme. Some will admit right away the rest will need convincing, time. One trick is to do a scheme before the feast/hunt or do with people you are already in an event with. Or young intellectuals who are good mentors, preferably genius with middle tree in learning.

The way multiple kids trait work is often total opposite on one trait so siblings already kinda 50-50, a zelous will refuse but if you somehow change it is hard to change back.

Not sure if only house counts, I was mainly playing more before the updates on branches so I had a huge dynasty and huge main branch by the time it even offered the option.