Returning a description based on a minimum and a maximum value
Perhaps instead of this convoluted branching, you could simply handle the three cases: whether there is a maximum and a non-zero minimum, a maximum and a minimum of 0 or there is no maximum.

private static Dictionary&lt;int, string&gt; humanReadableNumbers = new Dictionary&lt;int,string&gt;()
    {
        {0, "zero"},
        {1, "one"},
        {2, "two"},
        //etc...
    };

public static string Execute(int minimum)
{
    return string.Format("{0} or more", CultureInfo.CurrentCulture.TextInfo.ToTitleCase(humanReadableNumbers[minimum]));
}

public static string Execute(int minimum, int maximum)
{
    return minimum == 0 ? string.Format("Less than {0}", humanReadableNumbers[maximum]) : string.Format("From {0} to {1}", humanReadableNumbers[minimum], humanReadableNumbers[maximum-1]);
}


With this method, adding extra numbers is very simple. I've additionally chosen to split the method into two methods, one for no maximum, and one for a maximum. I feel it shows the different use-cases better, but if it is too much work to refactor all uses of it, this will work instead:

public static string Execute(int minimum, int? maximum)
{
    if(maximum.HasValue())
    { 
        return string.Format("{0} or more", CultureInfo.CurrentCulture.TextInfo.ToTitleCase(humanReadableNumbers[minimum])) 
    }
    else
    {
        return minimum == 0 ? 
            string.Format("Less than {0}", humanReadableNumbers[maximum]) :
            string.Format("From {0} to {1}", humanReadableNumbers[minimum], humanReadableNumbers[maximum-1]);
    }
}