When the limit (50) is exceeded, the user trying to send notifications should be informed about that during preview.
Description
Description
Related Objects
Related Objects
Event Timeline
Comment Actions
It's documented at https://www.mediawiki.org/wiki/Manual:Echo#Technical_details .
It's hard-coded (now to 50, but it will take the standard time to roll out). See T110306: Increase number of mentions allowed in a single post.
Comment Actions
Oddly, there is a separate config for Flow ($wgFlowMaxMentionCount) (not hard-coded).