hello,

please tell me how much keyword density is excellent or good for a any single keyword on a webpage.

There's no perfect formula, so don't worry about it.

Write your pages so that they make sense to your human visitors, and include a few synonyms / related keywords for the sake of the crawlers.

Keyword density has so little to do with your website ranking.
You could get high rank in a keyword, in fact, without using it on the page...

Keyword density is one of the biggest myths out there now. It's really not a factor anymore whatsoever. In fact, the last time I actually measured the keyword density of a page was about 3-4 years ago, maybe longer ago than that.

If you feel you absolutely must get an average then search for your phrase in the search engine--measure densities of the pages in the top 10 results and get an average density percentage. Then make your page "about" that keyword density percentage.