Grid format on archive pages

I have a question regarding archives page. I am trying to display the content on the archive page only in grid format with 2 or 3 columns.

I tried wrapping the ‘if have post stuff’ with a div and adding a css to the style.css file.

On the archive page I have:
<div class=”new-wrapper”>
…the post code…
</div>

On the style.css I added:
.new-wrapper { float: left; width: 200px; height: 200px; margin: 0px 20px 20px 0px; }

The problem is this is creating just one columns and I would like to have 2 or 3 columns.

Can you please advise on what to do in order to display the posts in the achive page only in grid format with 2 or 3 columns?

Thank you so much in advance.