Almost guessed it. We're reworking Imps and Ogres (useless meat) + adding units from other factions.
For example, 35 Pikemen is equal to 10 Demons.
50 Demons (Goats) is 35*50 XP = 1750

Ogres 40*13 = 520
+
Imps 150*4 = 600
= 1120
=> 32 little goats + there were about 10 of your own = 42

I think it's not difficult to find even more meat to feed them (Trolls, Harpies, Pikemen, Marksmen, Gnolls, Lizards, Centaurs, Gnomes). For example,

For reference, one Pit Lord raises 50 XP of Goats, meaning
K (number of Pit Lords needed to raise Demons) = XP (of the killed meat) / 50

If this number is greater than the number of Pit Lords you have, you need to reduce the stack for feeding so that they match exactly.

K (number of raised Goats) = XP (from feeding) / 35
At the same time, the number of raised Goats will not be greater than the number of units in the dead stack that was used for feeding.

For simplicity, 7 Pit Lords raise 10 Demons, meaning 7 (Pit Lords) * 50 (XP raised by one Pit Lord) = 35 (HP of a Goat) * 10 (Goats)