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).
Around half of the top 250 are participating atm https://save3rdpartyapps.com/ which I think gives a better idea of how big it is. Despite that, I think it won't be enough to change anything. Maybe if we got all top 250 subs to participate and extended the protest to being indefinitely.
I brought up the percentage of subreddits, which does matter, because it determines how many subreddits remain to fall back to. The activity level of the subreddits who are blacking out, however, doesn’t matter for that, and it was pb7 who brought that up, not me.
I stopped counting at lower subbed communities (below 100k IIRC), seeing diminishing returns. If they all add up that much it's interesting, as many of those communities are very niche, so may have a higher weight of unique subs.
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).