Use copyindex to create multiple VNets and subnets in ARM template .json

{
  "$schema": "https://schema.management.azure.com/schemas/2019-04-01/deploymentTemplate.json#",
  "contentVersion": "1.0.0.0",
  "parameters": {
    "vNetNames": {
      "type": "array",
      "defaultValue": [
        "private",
        "internal",
        "public"
      ]
    }
  },
  "resources": [
    {
      "type": "Microsoft.Network/virtualNetworks",
      "apiVersion": "2021-08-01",
      "name": "[parameters('vNetNames')[copyindex()]]",
      "location": "[resourceGroup().location]",
      "properties": {
        "addressSpace": {
          "addressPrefixes": [
            "[concat('10.', copyIndex(), '.0.0/16')]"
          ]
        },
        "copy": [
          {
            "name": "subnets",
            "count": 2,
            "input": {
              "name": "[concat('subnet', copyIndex('subnets'))]",
              "properties": {
                "addressPrefix": "[concat('10.', copyIndex(), '.', copyIndex('subnets', 3), '.0/24')]"
              }
            }
          }
        ]
      },
      "copy": {
        "name": "vnetcopy",
        "count": "[length(parameters('vNetNames'))]"
      }
    }
  ]
}