Numbers fall into the category Quantifiers (Click here).
In your example There are 5 pens, the word 5 functions as an adjective. We know this because it modifies--tells us more about / reduces the semantic scope of the noun pens.
In answer to your question "why is it some numbers sound like nouns when the really are just descriptive words?", for example:
Substantive: One hundred are in every package.the phrase One hundred is short for One hundred pens. It's a quantifier in form (that's what it looks like) but it has become a noun in function (that's its job in the sentence) because its noun has been omitted for the sake of efficiency: it's quicker to say One hundred than one hundred pens.