I've only made stabilized whipped cream to trim cheesecakes, I've never used it to cover an entire cake.
How much heavy cream will I need to make enough whipped cream to fill and cover a 2-layer 8 inch round cake?
You should have enough if you whip a quart (4 cups). Probably have enough left to pipe borders.
4 cups is a LOT to cover an 8" round...you could probably get away with 3 cups. (probably doesn't change your shopping list, but you can always use the cream for a different recipe!)