Create EKS cluster.

eks-demo.tf

terraform {
  required_version = ">= 0.12"
}

resource "aws_vpc" "eks-demo" {
  cidr_block   = "172.17.0.0/16"

  tags = {
    Name = "eks-demo"
  }
}

# Fetch AZs in the current region
data "aws_availability_zones" "available" {
}

resource "aws_subnet" "private" {
  count             = 3
  cidr_block        = cidrsubnet(aws_vpc.eks-demo.cidr_block, 8, count.index)
  availability_zone = data.aws_availability_zones.available.names[count.index]
  vpc_id            = aws_vpc.eks-demo.id
  tags = {
    Name = "eks-demo"
  }
}


data "aws_subnet_ids" "example" {
  vpc_id = "${aws_vpc.eks-demo.id}"
}

data "aws_subnet" "example" {
  count = 3
  id    = "${element(data.aws_subnet_ids.example.ids[count.index])}"
}

output "subnet_cidr_blocks" {
  value = ["${data.aws_subnet.example.*.cidr_block}"]
}


# data "aws_subnet_ids" "example" {
#   vpc_id = "${var.vpc_id}"
# }
#
# data "aws_subnet" "example" {
#   count = "${length(data.aws_subnet_ids.example.ids)}"
#   id    = "${data.aws_subnet_ids.example.ids[count.index]}"
# }
#
# output "subnet_cidr_blocks" {
#   value = ["${data.aws_subnet.example.*.cidr_block}"]
# }



module "eks" {
  source  = "terraform-aws-modules/eks/aws"
  version = "5.1.0"
  # insert the 3 required variables here
  cluster_name = "eks-demo"
  subnets      = ["subnet-abcde012", "subnet-bcde012a", "subnet-fghi345a"]
  vpc_id       = "${aws_vpc.eks-demo.id}"
}

For changing that count to any number, we can pass variable over there as var.count, and that variable value will come at time of
Terraform apply -var count=4 or any value

Same we can do for clusterName by passing variable from outside