I consider such numbers with a pinch of salt, the old problem of antiquarian authors deliberately inflating numbers in order to excite their readers.

Im no authority on the Pontic forces of Mithridates all I know is that against Sulla he fielded a more traditional Hellenistic force centred around a phalanx (Tigranes did the same against Lucullus I believe). Against Lucullus and Pompey however Mithridates fielded a much for Roman-esque force likely heavily composed of Theuropheroi and Thorokatai. He also had some heavy Galatian infantry, at least in the last Mithridatic wars.