"I am going to need some pens."
Translation:Voy a necesitar algunos bolígrafos.
Creo que Duo está equivocado y unos bolígrafos es una respuesta adecuada. Normalmente algunos y unos son intercambiable.
I had this same question, so I Googled it. It seems yes, "unos" or "algunos" are pretty interchangeable. There are some subtle differences, tho. "Unos" are about specific items, "algunos" are general items.
So maybe in this sentence, without context, "algunos" makes more sense. (I'm going to need some pens, any pens). But you could use "unos" if you were going to need some specific pens.