an example of using count to create N number of subnets without a lot of duplication

main.tf

module "public_subnets" {
  source = "modules/aws_public_subnet"

  environment        = "${var.environment}"
  vpc_id             = "${module.vpc.vpc_id}"
  vpc_cidr           = "${module.vpc.vpc_cidr}"
  cidrs              = "${var.public_subnets}"
  availability_zones = "${join(",", data.aws_availability_zones.available.names)}"
}

module.tf

resource "aws_subnet" "public" {
  count = "${length(split(",", var.cidrs))}"

  vpc_id = "${var.vpc_id}"

  # Determine the Availability Zone and CIDR block for this Subnet instance
  availability_zone = "${element(split(",", var.availability_zones), count.index % length(split(",", var.availability_zones)))}"
  cidr_block        = "${element(split(",", var.cidrs), count.index)}"

  # Default to creating public ip addresses for instances in this Subnet
  map_public_ip_on_launch = true

  tags {
    Name        = "public-${var.environment}.${element(split(",", var.availability_zones), count.index )}"
    Environment = "${var.environment}"
    Service     = "public subnet"
  }

  lifecycle {
    create_before_destroy = true
  }
}

variables.tf

public_subnets = "10.241.10.0/24,10.241.11.0/24,10.241.12.0/24"

#a better way with a map, but the logic above doesn't reflect it
variable "public_subnets" {
  type = "map"

  default = {
    prod           = "10.241.10.0/24,10.241.11.0/24,10.241.12.0/24"
    dev            = "10.242.10.0/24,10.242.11.0/24,10.242.12.0/24"
    stage          = "10.243.10.0/24,10.2413.11.0/24,10.2413.12.0/24"

  }
}