There is no way to pass parameters to exif metadata messages. We need to (possibly?) fix this in BitmapMetadataHander.
The problem is, that the metadata handler takes an array of message key for name of metadata field => expanded version of value. The value part is expanded in FormatMetadata long before the name part is in MediaHandler::addMeta.
Its not clear how to easily pass the number of items to the name field without doing a lot of restructuring.