// @nuget: HtmlAgilityPack

using System;

using HtmlAgilityPack;

​

public class Program

{

    public void Main()

    {

        var caso1 =  @"<p><br></p>";

        var caso2 =  @"

            <p><br></p>

            <p><br></p>

            <p><br></p>";

        var caso3 =  @"<p><br><p>hola</p></p>";

        Console.WriteLine(checaHtml(caso1)); //ok

        Console.WriteLine(checaHtml(caso2)); //tratar

        Console.WriteLine(checaHtml(caso3)); //ok

    }

    public int checaHtml(string html) {

        var htmlDoc = new HtmlDocument();

        htmlDoc.LoadHtml(html);     

        var elem = htmlDoc.DocumentNode.SelectNodes("p/br");

        if (elem != null){

            return elem.Count;

        }

        else {

            return 0;

        }

    }

​

}