gnus-large-ephemeral-newsgroup

gnus-large-ephemeral-newsgroup is a variable defined in `gnus-group.el'.
Its value is

200

Documentation:
The number of articles which indicates a large ephemeral newsgroup.
Same as `gnus-large-newsgroup', but only used for ephemeral newsgroups.

If the number of articles in a newsgroup is greater than this value,
confirmation is required for selecting the newsgroup. If it is nil, no
confirmation is required.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.