I tallied (roughly) the number of subscriber accounts, across all these reddits. It's around 1 billion. Obviously, it's not conservative as many will have subscribed to multiple. But, I see MAU (monthly active users) is about 1b. So, this is huge.
EDIT: Seems the source for MAU I found was way high, making this potentially even bigger. Every other source I've found is 400-500m range for MAU on reddit (as another pointed out).