"Les" is the definite article and means "the", whereas this sentence is indefinite (any five pens rather than five specific pens), so the article isn't required. If this where to be definite ("the five pens"), the order would be "Les cinq stylos" not "cinq les stylos" (which would be like saying "five the pens")

Numerals are determiners in their own right. They are used instead of indefinite articles.

"un stylo" = a/one pen
"cinq stylos" = five pens

You may add a definite article for specificity:

"les cinq stylos que j'utilise" = the five pens I use.