How should I come out as poly to my parents?

Try being honest and open with them and be willing to answer any questions that they have to reassure them that you are happy with how you are living

You can choose a quiet moment when you're sure you'll have all the time you need, then ask them to listen to you and tell them you'll be open to answer all their questions afterwards. Be honest about what this means to you and be there for them to help them understand if they have a hard time coming to terms with it.