The number on the page is a bit misleading. If you comb through the website's code on GitHub, you'll see that the 8,838 is actually the number of subreddits that agreed to participate in the blackout.

Calling it a flop isn't accurate either, though.

Sure, most subreddits don't care, but the largest and most active subreddits are overwhelmingly in support of the blackout, but they are also much more affected by Reddit's changes than smaller subreddits.

EDIT: Some words for clarity.

Reddit has more than 100k active subreddits lol. Also, this number doesn't mean much because community sizes vary a LOT