A small portion of a tribe fighting from chariots with javelins does not compare to 9000 mounted archers...

Romans had plenty of experience in dealing with javelin-armed cavalry and their armies habitually included plenty of their own. Caesar used Gauls, Germans and Numidians against the Gauls. (Maybe Iberians too, I forget.)

The majority of the Celtic armies still fought on foot and it was the heavy infantry clash which generally decided the battles between Romans and Gauls. Again, no real comparison to a fully mounted army like the Parthians fielded.