"smuk" is singular, "smukke" is plural. Examples: En smuk pige. A beautiful girl. (singular, common gender) Smukke piger. Beautiful girls. (plural) Et smukt hus. A beautiful house. (singular, neuter) Smukke huse. Beautiful houses. (plural)

"Smuk" = "beautiful" - singular, common gender (nouns with the article "en"). "Smukke" = "beautiful" - plural and also used attributively to singular nouns.