Q:

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. 

A:

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

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

