Are you asking about the number of people that belong to Causa Real or the number of people that usually attend the crongress?
In any case, I don't know any of the numbers, sorry. But there are for sure many more members that people in Portugal may imagine.
Causa Real is constituted by 11 Reais Associações (Royal Associations). Each Real Associação can represent a city (in case of Lisbon or my city Viseu) or a region (like Baixo Alentejo) depending on the number of members and other criteria. If you want to be a member of Causa Real you have to contact the nearest Real Associação. I searched on their website and couldn't find information about the number of members but here (
LINK) you have a list of the people who belong to the governing bodies and you see that's quite a lot of people.
Not all the members attend the congress of course. I actually think only a few of them do. Perhaps people from the governing bodies and not more