CountItems.cs

using System;
using System.Linq;
using System.Net;
using System.Text;
using Newtonsoft.Json.Linq;

namespace ConsoleApp
{
    internal static class Program
    {
        private const string ApiKey = "YOUR API KEY";
        private static readonly WebClient HttpClient = new WebClient {Encoding = Encoding.UTF8};

        private static void Main(string[] args)
        {
            const int itemId = 19721; // Pile of Ectoplasm Id number
            var itemCount = 0;

            itemCount += CheckCharactersInventory(itemId);
            itemCount += CheckMaterialStorage(itemId);

            Console.WriteLine(itemCount);
            Console.ReadLine();
        }

        private static int CheckCharactersInventory(int itemId)
        {
            var url = $"https://api.guildwars2.com/v2/characters?ids=all&access_token={ApiKey}";

            // Find and count itemId in all character's inventories:

            var json = HttpClient.DownloadString(url);
            var characters = JArray.Parse(json);

            return characters.Sum(character => (
                    from bag in character["bags"]
                    where bag.HasValues
                    from item in bag["inventory"]
                    where item.HasValues
                    where (int) item["id"] == itemId
                    select (int) item["count"]
                ).Sum()
            );
        }

        private static int CheckMaterialStorage(int itemId)
        {
            var url = $"https://api.guildwars2.com/v2/account/materials?access_token={ApiKey}";

            // Find and count itemId in your material storage:

            var json = HttpClient.DownloadString(url);
            var materials = JArray.Parse(json);

            return materials
                .Where(material => (int) material["id"] == itemId)
                .Sum(material => (int) material["count"]);
        }
    }
}