1,5552 sekundy googlování:

http://dotnetperls.com/count-array

using System;
using System.Collections;

class Program
{
    static void Main()
    {
        ArrayList ar = new ArrayList();
        ar.Add("One");
        ar.Add("Two");

        // Count the elements in the ArrayList.
        int c = ar.Count;
        Console.WriteLine(c);
    }
}