You can use the API https://developers.freshdesk.com/api/#list_all_ticket_notes to list all the conversations of a ticket. You can make use of a script to fetch the conversations of all the tickets as required. 


To know the tickets in which there are multiple conversations you can take an export of the tickets from the list view page. Choose the parameter 'Customer interaction' and if this is more than 1 it means the customer has replied to the ticket after creating it.