main.tf

provider "aws" {
  region  = "us-east-1"
}

module "module1" {
  source = "./module1"
}

module "module2" {
  source = "./module2"
  number = "${length(module.module1.azs)}"
}

module1.tf

data "aws_availability_zones" "available" {}

output "azs" {
  value = ["${data.aws_availability_zones.available.names}"]
}

module2.tf

variable "number" {}

resource "aws_security_group" "sgs" {
  count  = "${var.number}"
  name   = "test ${count.index}"
  vpc_id = "vpc-xxxxxx"
}