For a battle that we know nothing about, some stuff is at last starting to come out of the woodwork about Chalons ( a lot of it on the SoA forum page http://soa.org.uk/sm/index.php?topic=217.0).
Firstly there is Jordanes, who tells us who was fighting on each side and about this enigmatic ridge. His numbers are obviously incredible though, and that’s a problem when you are trying to wargame a battle using rules that have a troop ratio.
Second is a panegyric of Sidonius that tells us that Aetius came north with no milites, only auxiliaries. Some interpret that as no legions, just auxilia; others more broadly as being no regular Roman troops, just mercenaries or household.
Thirdly, we have the record that the Vandals (and some Alans) when they crossed into Africa under Gaeseric were counted and numbered 80,000 people in total, so we could guess than only 16-20,000 of those would be men capable of fighting. This is accepted as a reasonable figure for a Germanic host of the Volkswanderung period.
Fourth, Duncan Head has mentioned a argument of Lindner that Hungary at the time of Attila couldn’t have supported the horses for more than 15,000 Hunnic cavalry.