What is the difference between count and for_each?

Both create multiple instances of a resource, but they track them differently.

• count takes a number and identifies instances by index, such as resource[0] and resource[1]. Removing an item from the middle shifts indexes and causes Terraform to recreate or destroy the wrong resources.
• for_each takes a map or set and identifies instances by stable keys, such as resource["web"] and resource["api"]. Adding or removing one key only affects that instance.

resource "aws_instance" "app" {
  for_each      = toset(["web", "api"])
  ami           = "ami-12345678"
  instance_type = "t3.micro"
  tags          = { Name = each.key }
}

Use for_each when identity matters, such as per-environment or per-name resources. Use count for identical, interchangeable resources or simple conditional creation with count = var.enabled ? 1 : 0. Note that for_each keys must be known at plan time.