Alphatian Army
by Giampaolo AgostaI'd start with a 1% circa, so about 82,000 troops. That's because Alphatia needs to be on par with Thyatis at least in numbers. Moreover, Glantri focus on troop quality to compensate for small numbers, while Alphatia typically fields large armies. I'd also consider local armies outside this 1%. The same is true of Glantri, since most mages have their own personal guard of 20-60 (usually) élite soldiers -- for a total of 43*40=1720 more troops (possibly less, I've just taken the average and multiplied by the number of nobles).
The following information is reported in Champions of Mystara.
Alphatian Man-of-war: 200 airmen, 400 marines; 41 guns
Princess Ark: 12 officers, 60 boltmen, 15 airmen, 10 gunners, 4 support personnel; 7 guns
Navy Ranks: Admiral, Vice Admiral, Rear Admiral, Captain, Lieutenant Commander, Lieutenant, Midshipman; Warrant Officer, Chief Petty Officer/Chief Boltman, Petty Officer/Lead Boltman; Able-bodied Boltman/1st-class Airman, Boltman/Gunner/Airman, Apprentice Boltman/Gunner/Airman.
In addition to Boltmen (and variants, all using wands), there are also Heavy Marines (fighters) used only in mass battles.
If we assume the Man-of-War and the Princess Ark as representative of the typical primary combat ship (total crew 600) and escort/reconnaissance ship (Frigate, total crew 100), and we assume sail ships (large and small, with total crews respectively 75 and 35) to be the only sea ships used, we can easily derive the composition of the Alphatian navy and sky navy. Smaller skyships (brig, total crew of 25) may also be used, but these would be conscripted ships, or courier/scout ships.
Man-of-War: 25
Frigates: 45
Brig: 50
Large sailing ships: 80
Small sailing ships: 140
Total crew: 25*600 + 45*100 + 50*25 + 80*75 + 14*35 = 27,240
Almost exactly 1/3 of the 82,000 total troops. Of these, officers and marines of the skyships are magic users and elves (clerics are also present as officers, usually), thus making for more than 15,000 spellcasting troops and officers, and just 12,000 non-spellcasting.
The army is likely to be divided as well in spellcasting and non-spellcasting troops.
Given the scarcity of non-spellcaster freemen, non-spellcasting troops are likely to be slave armies led by officers of servant status, while high officers will be gentry or spellcasting nobles.
Spellcasting troops might be quite common, since 20% of the Alphatians are spellcasters, and not all of them can reasonably be supported by the slave population as idle nobles.
The typical Alphatian army would have a main body of spellcasting troops, usually taking the place of archers or crossbowmen in mundane armies. Gaz 2 implies that Alphatian spellcasting troops are not mounted (dragoons are possible and even likely in the mainland), which is also in line with the imagery (similar to that of British Redcoats).
Thus, non-spellcasting troops will generally fall in one of three roles: heavy infantry, pikemen, or cavalry.
Of these, IMO the presence of a specific corps of Heavy Marines implies that heavy infantry is otherwise scarce in the Army -- otherwise, heavy infantry from the army would fill in. Thus, I'd reserve heavy infantry for "crack troops", experienced fighters such as the guards commanded by General Torenal. This is also reasonable if you think that non-spellcasting troops are likely to be considered expendable, and heavy infantry are costly to equip.
Pikemen are a good option for expendable troops, making a good screen for spellcasters against cavalry charges. Most of the slave troops are likely to be pikemen, with little in the line of armour and weapons (probably leather armour, pike, short sword and maybe a buckler) and training. Promising slaves are probably moved early on to heavy infantry troops, and given the high casualty rate of pikemen, most will be Normal Men rather than Fighters.
Cavalry is likely to be reserved for pursuit. Thus, it must be mobile. Light cavalry is likely to be the primary choice, and cavalrymen are probably professionals (Servants) rather than slaves.
As for officers, army ranks gathered from the PWAs include: High General, General, Lieutenant General, Adjutant, Commander, Captain, Lieutenant, Sergeant.
By comparison with navy ranks, High General should be equal to Admiral, while Lieutenants should be the same rank in army and navy. Therefore, we need to add one more rank to make the two hierarchies equal (which is likely). A distinction between two types of General may therefore be in order (to make three ranks of General, in parallel with the three ranks of Admiral).
The three uppermost ranks (and likely also the Adjutant) are reserved to spellcasters (thus Broderick is at the top of his career, being a Commander), except in the Imperial Guard (Torenal's unit).
NCOs and troops probably also include more structure than just sergeant and private, e.g.: sergeant major, staff sergeant, sergeant; corporal, private 1st class, private and recruit.
The following structure can be derived, mixing the information from the PWA with elements of what is known for the fleet:
Army: 5300 (4 Divisions, commanded by a High General, with a General aide and assorted staff complement)
Division: 1300 (2 Regiments, commanded by a General with a Adjutant aide and assorted staff complement)
Regiment: 625 (4 Companies, commanded by a Lieutenant General, with a Commander aide and assorted staff complement)
Company: 150 (4 Platoons, commanded by a Captain, with a Cadet aide)
Platoon: 35 (3 Squads, commanded by a Lieutenant, with a Sergeant Major aide)
Squads: 10 soldiers (of various ranks) and 1 Sergeant or Staff Sergeant
The assumption is that each Kingdom has its own Army, led by a High General.
The Imperial Army probably follows the same structure, but is uses half-strength Armies (the idea is that, in wartime, armed forces from the kingdoms are integrated into the Imperial Army, and replaced by reserve divisions in their standard duties), dividing its forces in 20 Armies, plus the Imperial Guard (2000 soldiers and officers).
In my opinion, the Army is likely to be the only compound unit -- divisions are already homogeneous units. A typical Army may be composed by one or two magic-using divisions, plus two or three "auxiliary" (non-spellcasting) divisions (at most one cavalry division).
Half-strength Armies will be composed by a single full magic-using division, plus two half-strength divisions of auxiliaries (one of pikemen and one of horsemen).
In peacetime, most General-rank commanding officers leave their duties to their second in command (so they can pursue magical research, political intrigue, or leisure).
The Imperial Guard should be a single Division composed by three Regiments (two mounted heavy infantry and one medium cavalry), plus a small élite unit of 75 men under the direct command of General Torenal.