If the recipe calls for two cups of sliced portabello mushrooms, how many ounces or pounds should I buy?

For mushrooms, I normally use an 8 ounce package for 2 cups sliced. In this case, the 8 ounces is a weight rather than a volume.If the recipe calls for two cups of sliced portabello mushrooms, how many ounces or pounds should I buy?
You could by an 8 ounce package, which has anywhere from 2-3 medium caps, or just buy them in bulk. Either way, 2-3 medium caps should be enought to give you two cups.