Is there a shorter/simpler version of the for loop to anything x times?

Well you can easily write your own extension method:

public static void Times(this int count, Action action)
{
    for (int i = 0; i < count; i++)
    {
        action();
    }
}

Then you can write:

10.Times(() => list.Add(GetRandomItem()));

I'm not sure I'd actually suggest that you do that, but it's an option. I don't believe there's anything like that in the framework, although you can use Enumerable.Range or Enumerable.Repeat to create a lazy sequence of an appropriate length, which can be useful in some situations.

As of C# 6, you can still access a static method conveniently without creating an extension method, using a using static directive to import it. For example:

// Normally in a namespace, of course.
public class LoopUtilities
{
    public static void Repeat(int count, Action action)
    {
        for (int i = 0; i < count; i++)
        {
            action();
        }
    }
}

Then when you want to use it:

using static LoopUtilities;

// Class declaration etc, then:
Repeat(5, () => Console.WriteLine("Hello."));

foreach (var i in Enumerable.Range(0, N))
{
    // do something
}

One can create an IEnumerable of Int32:

Enumerable.Range(0, 10);

The ForEach extension method is also widely known (although not shipped with .NET). You could combine the two:

Enumerable.Range(0, 10).ForEach(index => ...);

So your example would become:

Enumerable.Range(0, 10).ForEach(_ => list.Add(GetRandomItem()));