The use of PLURAL in this message is inadequate and results in a nonsensical message if $1 equals 1.
If $1 is always greater than 1, then the use of PLURAL should be removed. If $1 may be 1, then the message should be as follows:
Enter the {{PLURAL:$1|name of one article that covers|names of up to $1 articles that cover}} the topics of your event.
{{PLURAL:$1||Each article should be on a new line.}}$1 is guaranteed to be a large number (in the hundreds). I thought this was already mentioned in the message documentation, but I just realized that it isn't. It's only documented for the campaignevents-worklist-error-too-large message (which uses the same quantity). I'll make a patch to update the docs.
If $1 is always greater than 1, then the use of PLURAL should be removed.
That's not a sufficient reason to remove PLURAL, see https://www.mediawiki.org/wiki/Manual:Messages_API#Be_aware_of_PLURAL_use_on_all_numbers.
Well actually, since it's just a qqq change, I've simply updated the message documentation on twn.