Do I get infinite troops?
 

Right from the start of the game, Peasants at the main Castle (Greenwort) were marked with the word "Horde" instead of a number that shows how numerous they are. I assumed that this means I get an infinite number of Peasants subject to leadership and money of course. After I defeated the Turtle, Swordsmen and Archers at the Greenwort Castle received the same "Horde" moniker, so I get an infinite number of those too.

I am wondering whether other troop types will also become infinite as I complete more quests and whether this will also occur in some other Castles or if it will be restricted to the main Castle in Greenwort.

You'll get infinite sea dogs (pirate brotherhood) and pirates (castle to the east of the pirate brotherhood) after completing some quests on the Freedom Isles. You will also get infinite horsemen from the main castle after the dwarf quest. Other than that I'm not sure.

Not only horsemen but also knigts, bowmen, swordsmen (and maybe also guardsmen) at different stages of the game IIRC.

And if you click on the units for sale with the left button (as you normally would) and then right-click on the unit portrait that appears in the purchase screen, you'll see the exact number of troops contained within the Horde.

you can also get infinite Sprites and Knights after defeating Korador. I don't know if it's random which troops will be marked as horde in castles after progressing with main quest so I can't assure you about those troops I mentioned.

Thanks for the answers!

You also get infinite Priests at the Knight's Temple

I got those upgraded Elves(Hunters?) infinite after Karador at the Elvish capitol.

Also be advised, that, every time the King promotes you, you have to check the various dwellings for new spells/troops! In my game, after signing the peace treaty with the dwarves, the King promoted me, and the mage school in Greenwort had 80 brand new Inquisitors! Now I have actually more inquisitors for recruitment than my leadership lets me recruit. Marvellous :) Also, they have a new spell now: Pygmy :(

using the trick to check the amounts I noticed the following:
1. When something becomes horde, it gets +9999 units. So if I had 25 archers left and then was promoted, the horde of archers will list 1024 units. Since I used my swordsmen up, it listed 9999.
2. No matter how many you buy from a horde stack, it never goes down.

Sacrifice+Resurrect (and Chargers)=infinite whatever troop for free :)