in a single edit?
Topic on Talk:Notifications
Most of the templates have a 5 or 10 parameter limit (e.g. this wiki's Template:Reply to has a 5 user limit, per instance).
Echo itself, currently has a 50-user limit. Any more than that, and the edit is assumed to be spam or an error, and no notifications are sent.
There are a few feature-requests for improving the current system, particularly Phab:T68078 and Phab:T108293.