You can do it using this macro
(defmacro normalize-list(l n)
"empties the list l when its size reaches N"
`(if(>=(length ,l),n)(setq ,l '())))
Examples
(setq slist'(foo bar baz)) ;->(foo bar baz)
(normalize-list slist 5);->nil
slist ;->(foo bar baz)
(normalize-list slist 3);->nil
slist;->nil