In Tableau desktop, I am bringing nearly 650k records, but when I count records in sheets, it is dropping around 6k records? It seems like it is not showing complete list of data. We do not have any filters. Any possible reasons?

Is it possible that some of those records have underlying rows of data which make up the aggregated view? In your system it might be counting all of those rows, whereas Tableau will display the aggregate count (a primary row of data made up of a subset of underlying rows of data).

Here's an example. This is a response by a fire department to a medical incident. In the view, Tableau will at the bottom left corner, display the number of marks and number of rows. Key word is 'display'. So, one would assume there are just two rows, two records.

When in fact, there are actually three rows of underlying data. The first unit responding was removed from the incident for some reason, then added back onto the incident, causing two rows of underlying data but aggregated to a single row in the actual 'display' result. The other unit only had one row of data. So a total of three (3) actual record rows, but only two (2) are displayed.

Hope that makes sense? Best, Don