I wanted to know that for multiple environments do I have to purchase an AMI separately or a single AMI would do.
Not quite clear on what you mean by "environment" in your question..
AMI is a single-node Teradata instance. CFT is also available to configure a multi-node instance.
An AMI is just a base disk image and metadata associated with the image. When you subscribe to an AMI in the AWS Marketplace, it allows you to launch the product associated with the disk image. When you subscribe to the Teradata Database AMI, you can launch between 1 and 32 instances (nodes) to make up a single Teradata Database system.
There is no cost associated with an AMI. Costs are associated with the number of instances you launch using the associated AMI. If you launch a single node system for Dev, a two node system for SIT, and a four node system for Prod, and you use the exact same instance type for each system, then your costs are (1 + 2 + 4 ) * ((Software + Hardware instance type cost) + (EBS Volume cost)).
Also, billing on AWS charges you on total usage, not separate systems. Billing will not show 3 different Teradata systems, it will show instance type in units of hour usage, EBS volume usage in units of months, etc. So if you launch Dev, SIT, and Prod systems, billing will not differentiate between the 3 systems.