Why not, "We have multiple potatoes?"
As a native speaker I would rather translate your sentence as Avem mai mulţi cartofi. Your answer is fine, though, you may want to submit it.
I guess if it isn't necessarily accurate to the way in which native speakers would say it, I wouldn't want to learn it that way.
Doesn't that mean: I have more potatoes?
"Avem" is "We have. "I have" is "Am" or "Eu am."