The Final Approach Tracks are (were?) calibrated to 15nm from touchdown. Traffic is (was) rarely turned on to intercept the localiser beyond 15nm (if only because it's slightly odd vectoring to a line which isn't there). With an average 3nm separation that makes 5 on final at best. If there are any more than five then the final is probably 'out too far'.
Try
https://www.adsbexchange.com/ , select Global radar View, select all trails and enable an EGLL-only filter. It will then depict all the well-worn grooves in the sky of the Heathrow Radar Manouvering Area. Then count the number on final!