Ash here from the Shopify team. Welcome to the Shopify Community!
I understand you're in need of your Merchant ID number for sales tax reasons. I'm happy to help out with this! The Merchant ID number associated with your account is something that our team can access from the back end. In order to share this information with you, I'll first need to get a bit more information from you. Since the information I require is sensitive in nature, I've sent you an email to the email address associated with your Shopify Community account so that we can discuss this more securely. Please reply to that email, and as soon as I hear back we can proceed with getting you the information you need.