using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        const string input = "{ lotsOfNumbers: [1, 1, 1, 1, 1, 2, 3, 5, 2394, 1] }";

        var deserialized = JsonConvert.DeserializeObject<MyShittyType>(input);

        // 5

        Console.WriteLine(deserialized.LotsOfNumbers.Count);

    }

}

​

public class MyShittyType {

    public ISet<long> LotsOfNumbers { get; set; }

}