The german entry says ca. 30.000 Romans....ok, Louis already looked that up, let me add that it says the goths made the roman cataphracts flee and then destroyed the roman infantry with their own cavalry. Don't know whether that's correct, but it's a possibility

I'd also like to add that the romans weren't all that good, after all they hid behind big walls in certain places because they were unable/afraid to go further.

But yeah, it seems quite hard to say how many they were, after all you can't count them anymore.