Is there any way to find out how many quickpost on a Group by user , is there any APi available for this or what are the tables we need to refer to get this output.
The answer for this should be as simple as using the Status Message Listing endpoint, available via REST, widget, or in-process APIs.
And the common technique for getting a full count of an item via a listing method is to request a page size of 1 and page index of 0. The result will include a TotalCount.
However, there appears to be an issue with this API, where it's not possible to filter by user and group at the same time, and I've logged this for review. It is possible to filter by user OR group.
As a workaround, you could use the search API, with the same approach of pagesize: 1, pageindex: 0 and TotalCount on the results.