using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var contas = new List<Conta>();

        contas.Add(new Conta("12345-9", 123.45m));

        contas.Add(new Conta("23456-8", 897.22m));

        contas.Add(null);

        contas.Add(new Conta("98873-7", 234.00m));

        contas.Add(null);

        contas.Add(new Conta("34343-3", 4141.98m));

        var contasNaoNulas = contas.Where(conta => conta != null);

        Console.WriteLine(contasNaoNulas.Count());

    }

}

​

public class Conta

{

    public Conta(string numero, decimal saldo)

    {

        Numero =  numero;

        Saldo = saldo;

    }

    public string Numero { get; set; }

    public decimal Saldo { get; set; }

}