variable "slave_account_id" {
 default = ["5686435678", "9889865446"]
}

provider "aws" {
  counter = "${length(var.slave_account_id)}"

  alias  = "aws-assume-${counter.index}"
  region = "eu-west-1"

  assume_role {
    role_arn     = "arn:aws:iam::${var.slave_account_id[counter.index]}:role/slave_role_for_master"
    session_name = "${var.slave_session_name[counter.index]}"
    external_id  = "EXTERNAL_ID"
  }
}

module "my_super_module" {
 counter = "${length(var.slave_account_id)}"
 providers = {
   aws = "aws.aws-assume-${counter.index}"
 }

 [...]
}