Code:
private void button_Click(object sender, RoutedEventArgs e)
        {

            int aantaltextBox = textBox.Text;
            int limiet = textBox1.Text;
            int aantallabel = label.Content=0;
            while (aantallabel < limiet)
            {
                label.Content = aantallabel + aantaltextBox;
            }
If both text boxes contain a number, there is no need to convert to integer because it has assigned itself as an int (or so it should be). You are also assigning it to an int variable.