Can we give serial numbers in views to each columns or row in a sequential manner? Can we give serial numbers in a views to each columns or row in a sequential manner from left to right(like:1,2,3----so-on) without any manual work in backend .

I am using views format "content" not "field" in views,so there is no option to add field view result counter in views,i am using view mode in views display.
When format is content

You can override views-view-unformatted.html.twig and perhaps create a twig file for just this particular view.  

If your view name is test you can copy from core/modules/views/templates/views-view-unformatted.html.twig to your THEMENAME/templates folder and rename to views-view-unformatted--test.html.twig.  In this twig file you can use {{loop.index}} inside the for loop to give you the row index/count sort of like this:

{% if title %}
  &lt;h3&gt;{{ title }}&lt;/h3&gt;
{% endif %}
{% for row in rows %}
  {%
    set row_classes = [
      default_row_class ? 'views-row',
    ]
  %}
  &lt;div{{ row.attributes.addClass(row_classes) }}&gt;
    &lt;span&gt;row count: {{loop.index}}&lt;/span&gt;    
    {{ row.content }}
  &lt;/div&gt;
{% endfor %}


When format is fields

Yes you can add view results counter when the format is fields