I have an app that search for tweets using particular hashtag by an user, but it returns only 100 records maximum, how can i get the actual count or all tweets.
The basic search API will return Tweets from within the past seven days, but the index may be optimised for recency and performance rather than for completeness. That said, you should also be aware that you can page through the results, so if you’re seeing 100 results (the maximum count you can specify per page) then you can also use the Working with Timelines documentation to page through the results if there are more than 100. The basic search API offers no way to get a simple “count”, you need to calculate this yourself.
Beyond that, for full index Twitter search you should look at the commercial Gnip Search products. The 30-day Search API may be a good fit to what you are trying to do, and also offers the ability to just ask for the count result rather than all the Tweets.