Mame files usually have one "working" version, and a lot of "dependent" versions. The working versions work on their own, while the dependent versions need the working version to be in the same directory to work. This is for old games, newer games actaully need other files called "CHD" files which contain all the game data in the same directory to work. CHD files are usually a lot larger than rom files, for instance rom files of one game might be a few kilobytes while the chd file could be bigger that 200 megabytes. The trick is to find the working roms and chd files. These are usually larger than their dependent counter parts and have more downloads, those are the metrics I use for finding them!