The merge feature was built with the purpose of reducing duplicated or surplus identities that are actually presenting the same identity.
There should be two or more identities in the merging process. After the merge is done there is only one identity left as a result. Such an identity that is the result of the merge collects and unites all the sightings and other relevant data from merged identities (first seen, last seen, number of sightings, last seen on...).
After the process, the user can no longer access the data of the merged identities in GUI nor can he see any permanent indicators that the merge was done. Merge works in a general and retroactive way.
The report page is showing that there were 50 unique visitors recorded in the last 24 hours before the merge occurred. Right after the merge, data gets synced and now the report page shows 49 unique visitors. A conclusion can be drawn that 1 unique visitor was removed in the merging process and the system has adapted.