Create a static integer field that keeps a count across all instances(objects) of a class. Create 3 or 4 object in derived class and then check total instances.
using System;
static class Program
{
    static void Main(string[] args)
    {
        Count a = new Count();
        Count b = new Count();
        Count c = new Count();
        Console.WriteLine(Count.count);
        Count d = new Count();
        Console.WriteLine(Count.count);
    }
}
public class Count
{
    public static int count;
    public Count()
    {
        count++;
    }
}
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