main.tf

resource "aws_vpc" "main" {
    cidr_block = "${var.main_cidr}"
    enable_dns_hostnames = true
}
resource "aws_subnet" "main" {
    vpc_id = "${aws_vpc.main.id}"
    cidr_block = "${var.main_subnet.cidrs[count.index]}"
    availability_zone = "${var.main_subnet.availability_zones[count.index]}"
    map_public_ip_on_launch = true
    count = "${length(var.main_subnet.cidrs)}"
}
variable "main_cidr" {
    description = "AWS vpc main cidr"
}
variable "main_subnet_cidrs" {
    type = "list"
}
variable "main_subnet_availability_zones" {
    type = "list"
}

terraform.tfvars

main_cidr = "172.17.0.0/16"
main_subnet.cidrs = ["172.17.0.0/20","172.17.16.0/20"]
main_subnet.availability_zones = ["us-east-1b","us-east-1e"]

これでvpc_subnetを参照するようなときに一つの値で記述できます。

# aws_elbのsubnetsとか
 subnets = ["${aws_subnet.main.*.id}"]
# autoscaling_groupのvpc_zone_identifierとか
 vpc_zone_identifier = ["${aws_subnet.main.*.id}"]
# countを使ったインスタンスのsubnetを交互に選択させるとか
  subnet_id = "${element(aws_subnet.main.*.id, count.index%length(aws_subnet.main.*.id))}"