jaikumar1, You can fetch maximum of 1000 records from the API. There is limit in API to give maximum results to page_size: parameter must be less than or equal to 1,000.
If you want to call data more than that you can use "page" parameter for next set of records.
Your API would be like below "?date_start=2018-10-18&date_end=2019-02-19&page_size=999&page=1" for next set of records after 999 you have to pass "page=2".
In your case might be results you are getting 100 is due to the particular date range selected.
Also I would recommend to showing no more than 25 threads per page. Increasing the page_size may cause performances issue.