Level 88
It's in the documentation:
Your merchant account ID is a unique identifier for a specific merchant account. Within your Braintree gateway, you can have multiple merchant accounts to process transactions for different businesses or currencies. If you have a single merchant account, it is not necessary to specify a merchant account ID in your API requests. If you have multiple merchant accounts and choose not to specify the merchant account ID, all requests will process through your default merchant account. Look here for more information about specifying merchant accounts in API requests.
So in most cases you won't even have to create such an account ;)