The Terraform Book

Chapter 3: Building an application stack with Terraform
Listing 3.71: Using ternary with count
variable "environment" {
default = "development"
}
resource "aws_instance" "web" {
ami
= "${lookup(var.ami, var.region)}"
. . .
}
count = "${var.environment == "production" ? 4 : 2}"
Here we’ve set a variable called environment with a default of development. In
our resource we’ve configured our count attribute with a conditional. If the var.
environment variable equals production then launch 4 instances, if it is the default
of development, or any other value, then only launch 2 instances.
The condition can be any interpolation: a variable, a function, or even chaining
another conditional. The true or false values can also return any interpolation or
valid value. The true and false values must return the same type though.
The condition supports a bunch of operators. We’ve already seen equality, ==,
and Terraform supports the opposite operator != for inequality. It also supports
numeric comparisons like greater or less, > and = and