The Terraform Book

Chapter 3: Building an application stack with Terraform
Listing 3.65: Interpolated math
tags {
Name = "web-${format("%03d", count.index + 1)}"
}
This would add one to every count.index value producing the tags web-001 and
web-002 respectively. We can do other math: subtract, multiple, divide, etc., on
any integer or float variables.
We can also iterate through list elements with count.index. Let’s create a list
variable with some tags we’d like to add to our instances.
Listing 3.66: AWS owner tags in variables.tf
variable "owner_tag" {
default = ["team1", "team2"]
}
We’d like to distribute our instances between these two tag values in web.tf.
Listing 3.67: Splitting up the count instances
resource "aws_instance" "web" {
. . .
}
tags {
Owner = "${var.owner_tag[count.index]}"
}
count = "${length(var.instance_ips)}"
Version: v1.1.1 (e771e4d) 53