How to get nic from azure dynamically and associate it to the loadbalancer in terraform

Hi all ,
I am trying to get the data source of network interface id dynamically as a variable and i want the code to iterate through the list and it should add itself to the backendpool association . I am able think of the below logic but am facing error like count can be used only when count argument is set , Please help me code is below

data "azurerm_network_interface" "example" {
 count               = length(var.nic_name)
  name                = var.nic_name[count.index]
  resource_group_name = "networking"

resource "azurerm_network_interface_backend_address_pool_association" "example" 
  network_interface_id    = data.azurerm_network_interface.example[count.index].id
  ip_configuration_name   = data.azurerm_network_interface.example[count.index].ip_configuration[0].name
  backend_address_pool_id =

variable "nic_name" {
type = list
default = [ "somenic1","somenic2"] 

Wondering how do you create those network_interfaces?