Calculate average from .txt file
Enclose the file reading in a using statement
Use double.TryParse or regex to parse the numbers
Use Linq extention method Average() to calculate average
The size of strInput is strInput.Count, no need for a loop 


Hear is a simpler code:

using (StreamReader bestand = new StreamReader(@"F:\RenoD_Oef_Strings\Punten.txt"))
{
    string strRow; // String used to read every row of .txt file
                    // Code
    while ((strRow = bestand.ReadLine()) != null) // Read row per row in .txt
    {
        List&lt;string&gt; strInput = strRow.Split(' ').ToList(); // String List used to store all data from .txt
        List&lt;double&gt; dblPoints = new List&lt;double&gt;(); // Double list used to store all values

        foreach (var x in strInput)  // Checks if elements in the list can be converted in to a double 
        {
            if (double.TryParse(x, out double result))
            {
                dblPoints.Add(result);
            }
        }
        double dblAverage = dblPoints.Average();

        Console.WriteLine(dblAverage.ToString());
    }
}

Console.ReadKey();