We have some limits in place elsewhere like maximum mentions that just provide informative feedback along the lines of “you’ve exceeded the maximum number of mentions so no one will be notified.”
Maybe that’s what we do here? Set some maximum bulk size for notifications and show that in a non-obtrusive way when it’s exceeded?
We could pick a reasonable default between 5 and 20. Sites that never want bulk actions to notify could set it to 0 and sites that always want to notify can set it to a very large number.