If I sub ground cardamom for whole, any idea how much I should use?

Agree with Stephanie Bourgeois...I cook with cardamom often (in all forms) and it's a strong flavor. I would start with 1/8 tsp (compared to the 1 tsp seeds in original recipe). You can always add more and mix in but it's impossible to take out too much.

You won't need quite as much as the original. I would start with 1/8 - 1/4 teaspoon for 2 pounds of sweet potatoes. You can always start with less and then add more after they have roasted if you'd like a strong flavour.