using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var tasks = new List<Task> {

            new Task { Assignees = new List<string> {"saurabh"}},

            new Task { Assignees = new List<string> {"saurabh", "manvel"}},

        };

        var assignees = tasks.SelectMany(t => t.Assignees);

        Console.WriteLine(assignees.Count());

    }

}

​

public class Task

{

   public List<string> Assignees {get;set;}

}