example.tf

resource "azurerm_resource_group" "test" {
  name     = "terraform-testing"
  location = "South Central US"
}

resource "azurerm_virtual_network" "test" {
  name                = "acctvn"
  address_space       = ["10.0.5.0/24"]
  location            = azurerm_resource_group.test.location
  resource_group_name = azurerm_resource_group.test.name
}

resource "azurerm_subnet" "test" {
  name                 = "acctsub"
  resource_group_name  = azurerm_resource_group.test.name
  virtual_network_name = azurerm_virtual_network.test.name
  address_prefix       = "10.0.5.0/24"
}

resource "azurerm_public_ip" "test" {
  name                         = "acctpip"
  location                     = azurerm_resource_group.test.location
  resource_group_name          = azurerm_resource_group.test.name
  allocation_method            = "Static"
}

resource "azurerm_lb" "test" {
  name                = "test-lb"
  location            = azurerm_resource_group.test.location
  resource_group_name = azurerm_resource_group.test.name

  frontend_ip_configuration {    
    name                 = "pip-config"
    public_ip_address_id = azurerm_public_ip.test.id
  }
}

resource "azurerm_lb_backend_address_pool" "test" {
  resource_group_name = azurerm_resource_group.test.name
  loadbalancer_id     = azurerm_lb.test.id
  name                = "backend-ap"
}

resource "azurerm_lb_nat_pool" "test" {
  resource_group_name            = azurerm_resource_group.test.name
  name                           = "ssh"
  loadbalancer_id                = azurerm_lb.test.id
  protocol                       = "Tcp"
  frontend_port_start            = 50000
  frontend_port_end              = 50099
  backend_port                   = 22
  frontend_ip_configuration_name = "pip-config"
}

resource "azurerm_virtual_machine_scale_set" "managers_vmss" {
  name                = "test-vmss"
  location            = azurerm_resource_group.test.location
  resource_group_name = azurerm_resource_group.test.name
  upgrade_policy_mode = "Manual"
  overprovision       = false

  sku {
    name     = "Standard_A0"
    tier     = "Standard"
    capacity = 2
  }

  storage_profile_image_reference {
    publisher = "Canonical"
    offer     = "UbuntuServer"
    sku       = "16.04-LTS"
    version   = "latest"
  }

storage_profile_os_disk {
    name              = ""
    caching           = "ReadWrite"
    create_option     = "FromImage"
    managed_disk_type = "Standard_LRS"
  }

  storage_profile_data_disk {
    lun            = 0
    caching        = "ReadWrite"
    create_option  = "Empty"
    disk_size_gb   = 50
  }

  os_profile {
    admin_username       = var.vm_user
    admin_password       = var.vm_pass
    computer_name_prefix = "terraformtest"
  }

  os_profile_linux_config {
    disable_password_authentication = true

    ssh_keys {
      path     = "/home/admin/.ssh/authorized_keys"
      key_data = file("~/.ssh/id_rsa.pub")
    }
  }

  network_profile {
    name    = "terraform-network-profile"
    primary = true

    ip_configuration {
      name                                   = "test-ip-config"
      primary                                = true
      subnet_id                              = azurerm_subnet.test.id
      load_balancer_backend_address_pool_ids = [azurerm_lb_backend_address_pool.test.id]
      load_balancer_inbound_nat_rules_ids    = ["azurerm_lb_nat_pool.test.*.id"]
    }
  }

  connection {
    type     = "ssh"
    host     = azurerm_public_ip.test.ip_address
    user     = var.vm_user
    private_key = file("id_rsa")
    ##Here's my issue:
    port     = format("5000%d", count.index) 

  }
}

variable "vm_user" {
  default = "admin"
  type    = string
}

variable "vm_pass" {
  default = "thisP@55word989876"
  type    = string
}