If you're doing it by spawning an ItemPickup prefab, you can change the amount on the pickup:
1. Get a reference to the ItemPickup component: var itemPickup = mySpawnedObject.GetComponent<ItemPickup>()
2. Get the item definition amounts: var itemDefinitionAmounts = itemPickup.GetItemDefinitionAmounts()
3. Modify your array: itemDefinitionAmounts[0].Amount = myAmount
(you would have previously saved the amount the enemy character had when it died)
4. Apply the changes: itemPickup.SetItemDefinitionAmounts(itemDefinitionAmounts)