How to fix Error: Invalid count argument - The count value depends on resource attributes that cannot be determined until apply in Terraform

# ❌ Wrong - depends on computed resource attribute
resource "aws_instance" "example" {
  count = length(aws_subnet.example.*.id)
  # ...
}

# ✓ Correct - use variable
variable "instance_count" {
  type    = number
  default = 3
}

resource "aws_instance" "example" {
  count = var.instance_count
  # ...
}

# Apply only the resource that count depends on
terraform apply -target=aws_subnet.example

# Now apply the rest - count can be calculated
terraform apply

# Instead of count with computed length
resource "aws_instance" "example" {
  for_each = toset(["server-1", "server-2", "server-3"])
  # ...
}

# subnets/main.tf - creates subnets
resource "aws_subnet" "example" {
  count = var.subnet_count
  # ...
}

output "subnet_ids" {
  value = aws_subnet.example[*].id
}

# instances/main.tf - uses output from subnets module
module "subnets" {
  source       = "../subnets"
  subnet_count = 3
}

resource "aws_instance" "example" {
  count             = 3
  subnet_id         = module.subnets.subnet_ids[count.index]
  # ...
}

# ❌ Wrong - data source depends on resource being created
data "aws_subnets" "example" {
  filter {
    name   = "vpc-id"
    values = [aws_vpc.example.id]  # Not known at plan time
  }
}

resource "aws_instance" "example" {
  count   = length(data.aws_subnets.example.ids)
  # ...
}

# ✓ Correct - data source uses variables
data "aws_subnets" "example" {
  filter {
    name   = "vpc-id"
    values = [var.vpc_id]  # Known at plan time
  }
}

resource "aws_instance" "example" {
  count   = length(data.aws_subnets.example.ids)
  # ...
}