I entered site:example.com on google search it gave me list of unique pages exists on my website, and then I selected 'Past Week' filter under 'Show search tools' it gives me just 4 results when I have more then 50 posts from past one week.
Sitemap.xml is submitted as well.
It's quite possible that those pages have not (yet) been indexed. Try a Google search for site:www.example.com/canonical/url to see if one of the missing pages is indeed in Google's index.
Submitting an XML sitemap is not a guarantee that those pages will be indexed - it is simply a recommendation.
If you look in Google's Webmaster Tools it should tell you (from your Sitemap) how many URLs have been submitted and how many have been indexed.
site: search is not necessarily reliable and all results may not be shown. Another point is how Google has determined the publish date (or last updated date (?) if looking at the advanced search page) of the page. Other dates that appear on page can affect this.
<changefreq> to determine crawl rates, but it might not (probably not IMO). It might use the <lastmod> as a factor in determining the last modified date of the page - but it certainly uses other metrics as well. I have a site where Google has picked up an arbitrary date that appears near the top of the page as the publish date of the page - it is not - and this differs from the <lastmod> date specified in the XML sitemap.