Is there some handy way to expire buy count of msgs

[Harry Putnam]

I'd like to be able to total-expire a specific group when it gets to
50,000 messages.

The common way I've been using is by number of days like this:
(setq gnus-parameters
        '(("_ex$"
    (total-expire . t)
    (expiry-wait . 18))
          ("_tr$"
           (total-expire . t)
    (expiry-wait . 90)
          (gnus-visible-headers
           (concat gnus-visible-headers "\\|^X-Spam-Report:.*_FARAWAY_"))
          ("_$"
    (total-expire . t)
    (expiry-wait . 340)))))

My best guess to end up around 50,000 msgs was the last one there to
hold msgs for 340 days.

It was not a good guess, and I'd rather have some way to do it by
number of messages rather than amount of time held. But still, expire
the oldest ones first.