Why we need sitemap for our website???
I use XML sitemap for all my sites. Through him you can add maximum 500 pages. Adding sitemap to google you are telling him that you have done something new and that he should come and look for it 
I add new Sitemap to google every few days.I hope this helps you
There is 500 limit on your sitemap; rather you are limited to 50,000 records in an XML file [per the schema] and using a tiered structure where the first file explains the others, you can run 50,000 parts, with 50,000 pages each.
If your tool limits you to 500 pages it’s time to consider a new tool.
A site map does not guarantee indexing of your web site; on the other hand, it ensures that if you have done your SEO right (consistent link-building and targeted web pages), a search engine spider will be more likely to index the whole website instead of doing it partially - in fact with the right format, you can almost ‘hypnotize’ the spider into doing a complete crawl.
Can you let us know that about which sitemap are you talking. There are two types of Sitemap, one is sitemap.xml which is for search engine and other is html sitemap which is for human being.
Yes of course. If a website has hundreds of pages, it can get rather chaotic in a navigation bar. Apart from this, both human browsers and search engine spiders need an easy access to find all the pages within a site. Therefore a sitemap is very essential. It gives them a simple navigational tool.
The original question has clearly been covered. Let’s avoid repeating the great answers already given and let the OP or someone else follow up with any new secondary questions they may have.
Thanks for sharing everyone.
sitemap will give the full details of your web pages to the search engines for indexing
To make it simple and understandable, think about a general map. What’s the use of map?
Of-course, it will guide a person how to go and where to go. Likewise, a sitemap tells the crawlers where a link is located so it can crawls/follows the link and index it. Generally we use two sitemaps: one is xml sitemap and other one is HTML sitemap.
XML sitemap is created for search engine spiders, so that they can crawl and index each and every link presented in sitemap. Second, HTML sitemap which is generally created for visitors to let them know about the link structure of the website.