The ~P format directive can do simple pluralization.

* (format nil "You have ~D goat~:P." 42)
"You have 42 goats."

* (format nil "You have ~D goat~:P." 1)
"You have 1 goat."

* (format nil "You have ~D fl~@:P." 42)
"You have 42 flies."

* (format nil "You have ~D fl~@:P." 1)
"You have 1 fly."

Irregular plurals are more complicated:

(format nil "You have ~D ~:*~[mice~;mouse~:;mice~]." n)

(Thanks to stassats for the flies and mice.)