Are merged tickets counted as one or two tickets in the total ticket volume?
Zendesk Explore considers unique ticket IDs when you report on the number of tickets. For example, if you use the metric Solved tickets, then the report will show two tickets as part of the volume of total solved tickets.
However, there could be different filters applied to the report that change the results displayed. For example, if the report uses the Ticket solved - Date filter and the non-merged ticket wasn't yet solved, the report will show only the merged ticket as part of the volume of total solved tickets.
For more information, see these articles: