In Risk, when your units are eliminated, can they be reused as reinforcements or do they die for good? Thanks!

Answer 1

the armys you lose when you roll against each other go back into your supply armies for when its you,re turn again and you get what ever armies you have earned either from the countries you own or the cards you turn in also if their are other colored armies that nobody else are using then you can use them also

Answer 2

They are gone forever. You can only get new armies by cashing in cards or by holding countries and continents.