The merged tickets would automatically get closed and would not be considered in any of the Reports. However, if you want to get the number of tickets that were merged, you can make use of the Observer rule to add a tag on merged tickets which could be later filtered on your list view page.
Move to Admin--> Observer--> New Observer rule.
Event performed by: Agent
Event Performed : Add note (Any)
Conditions : Last interaction contains "This ticket is closed and merged into ticket"
Action : Add Tag - Merged
Once this rule is set, for all merged tickets, the tag will get added and you can filter the Secondary ticket from the list view directly.