Q:

For an HTML element say div has n number of classes , how does it affect the render tree and how is the performance

<div class="a b c d e f g h i j">
</div>

The browser parses the data to construct DOM and CSSOM .
The CSSOM and DOM trees are combined into a render tree, which is then used to compute the layout of each visible element and serves as an input to the paint process which renders the pixels to screen. Optimizing each of these steps is critical to achieve optimal rendering performance.
So will having n number of classes have any impact on render tree construction.

A:

The browser searches the DOM for matching CSS rules, then get rendered by the browser. Theoretically, it would take longer if the DOM has to search lots of style sheets and classes for each element to find a match, but that may not all be practical nowadays with faster and more efficient processors and browsers. There is an article floating around the web that you may be interested in: Perfomance Impact of CSS Selectors. Hopefully it answers your question. 
I would just optimize the CSS the best you can.

