When we export a CSV file for a search query, the exported file contains only 500 rows, even though the search result contains more than 500 rows. That's default behavior.
In order to list more than 500 rows, first we need to increase the MaxRowFetchCount parameter value in Platform Server. Please refer the following article that explain how to set this parameter:
Once the MaxRowFetchCount has been set to a higher value, the exported CSV file then won't be limited to 500 rows.