During the first 30 days of your Azure free account, you can spend up to USD200 without incurring any costs. You can create as many instances of Azure Digital Twins (ADT) as you want, as long as you stay within the service limits (10 instances per region per subscription).
Because ADT is not one of the free services of the Pay-as-you-go subscription, after the 30 day period, you will be billed for the use of ADT. Because this is billed in a pay per use fashion, you will only be billed when you use the product. See more about the pricing details for ADT here.