As part of the "Validate Matches" screen it may be pertinent to also have the number of records that ran through the MatchMaker engine. I'm not sure, maybe this was intended to be a part of the Match Statistics that is not yet available?
However, the validation screen is showing you a "match pool" and not the results of a single run of the matching engine. So there is not any particular row count associated with what you see in the match validation UI.
One place where a row count would make sense would be in a log of all the times and dates the match engine has been run. I'm not sure there is such a log at the moment, but when we do implement that, we'll be sure to include row count.
The Match Engine log should now display the total number of matches that are in the match pool at the end of each runl, and the number of matches each munge process found.