gnus-keep-backlog

gnus-keep-backlog is a variable defined in `gnus.el'.
Its value is

20

Documentation:
*If non-nil, Gnus will keep read articles for later re-retrieval.
If it is a number N, then Gnus will only keep the last N articles
read. If it is neither nil nor a number, Gnus will keep all read
articles. This is not a good idea.

You can customize this variable.