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.

Less than 2% of subreddits but what percentage of total activity?

Not total activity, but I think half of the top 250 subreddits participating in the protest gives a good idea https://save3rdpartyapps.com/

That doesn’t matter for the question of whether users will fall back to visiting other subreddits.

So why did you bring it up?

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.

If I frequent /r/financialindependence, I’m not going to suddenly switch over to /r/guineapigs, I’ll just go do something else with my time.

A portion of users will do exactly that, because Reddit is just a way for them to pass some time.