There more than a few blurry spots in our knowledge of the 6thC BC, there could be a lost king whose name has not survived and perhaps whose deeds have been assigned to another, in any event, I dislike pretending to be a dead fellow. (Rosius is presumably sleeping somewhere in a cave until the call comes...)
Rosius the Forgotten, showing the effects of 30 years in the field. Oddly his personal standard appears to be nearly identical to that once carried behind Queen Johanna of Valdur. (Hinchliff Sassinid and Minifig Seleucid figures)
A few more things need to be considered before we get to a list of units, how big is the army going to be and how is the army theoretically raised and maintained so that if and when a major campaign is launched, there'll be a basis for replacing casualties (Please note that I make no reference to paying for said army, I find financial matters tedius enough in this world).
In reverse order, lets look at how the army is raised and please do not confuse this with the historical Persian army. The core of the army, both native Medes and subject regiments are raised by district. Young men are selected each year to serve for 4 years, should they live so long. Each winter an order is sent home as to how many recruits are required to bring the unit up to strength once the full timers have been dismissed. Assuming that there are enough young men to replace not only the released men but also any sick. wounded and dead soldiers, the units are thus brought up to full strength by the start of the campaign season. The soldiers who have served their time, return to their homes but remain liable for call up with each district being resposible to furnish a 2nd full strength unit, normally for home defence. Most districts supply what we may call line infantry, equipped according to their native fashion, but some districts supply cavalry.
In addition to the district regiments, the pick of the young noblemen of the kingdom are drawn off to serve in the King's guards, both horse and foot. Once they have served their 4 years, most return home but some are picked for long time service as senior oficers in the district regiments or as members of the palace guard which does not normally take the field. Casualties to the guards are immediately made good by other regiments in the army. There are also some small technical units which are maintained directly by the king on a full time basis, the Corps of Charioteers, the Department of the Quartermaster, and the Engineers.
There remain two other sources to be considered, on occasion, contingents from allied kingdoms may be found fighting alongside the Medes or even under their command and, when needed, mercenaries may be hired. This is normally done as a way of obtaining specialist troop types not available in the district regiments or during times of political unrest. Mercenary units do not have a regular system for replacing casualties and may be brought up to strength periodically or left to dwindle away and be repalced by a fresh unit as expedient.
Apulian mercenaries in Persian service? Who knew?
Now we know how the units are raised, how many should there be? I'm certainly not going to paint even 120,000 Medes at 1:50 which leaves me various options but the one I am going to choose is to fight the campaign with those units that I can field even though they will never all be on the table at once and even though this means I will be fighting a major war of with an army of almost certainly less than 20,000 men.
The army, following its reorganizaation, and omitting any Greek or Bythnian units, currently has 4 regiments of cavalry, 9 of infantry plus a small contingent of chariots and camelry. The Immortals are actually a double sized unit which is fielded as wings and some of the light infantry regiments are currently understrength and currently take the field in combined units. The cavalry has never been properly reorganized and are also currently fielded as composite regiments. This is probably as big an army as I would ever want to put on the table at once but there are some more units that I want to build, like Persian infantry in long robes, and of course, the composite regiments need to be split and filled out or 1 of the units scrapped. Its a rare table top teaser that has as many as 16 units and of course, in a 'proper' campaign, the size of a force will be determined by the events leading up to a battle. So if I plan on 6 cavalry units, counting the chariots and cavalry as one, and 14 infantry units, then I should be all set.