gnus-large-newsgroup

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

200

Documentation:
*The number of articles which indicates a large newsgroup.
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.

Also see `gnus-large-ephemeral-newsgroup'.

You can customize this variable.