There are similar number for goblin trolls, ogres, beak dogs, and blizzard men: they seem to be 25XX (I don't remember what XX were, but it totals to a bit over 2500), and I haven't seen the number decreased as they've lost them on raids against me.
I fixed that problem, turns out that without [ANIMAL] tags (particularly [ANIMAL_ALWAYS_AVAILIBLE] referencing the particular creature) & physically making trolls inedible by changing the body definition, the aveage Troll drops
40 meat worth in muscle therabouts making them ultra nutrious per every troll.
1 troll is killed for instance in the goblin tower, that's 40 meat, every surplus allows the population to expand. Trolls are also dipping into that and eating due to buggy semi sapience simulations. This leads to the population explosion because its not controlled in any manner and they eat a concurrent diet of beak-dogs & trolls of which are both common domestic in the basic vanilla raws.
[ANIMAL] defintions should be default vanilla because Toady's hit a major roadblock here.
Also
i covered it here, but this also explains horsesplosions, a condensed explanation would be that if civ's didn't focus on the most meat productive animals and instead had new behaviours like diverting crops INTO grazers and misc animals like pigs to fatten them up, they'd need to rely less on a bulk of animals and could maintain more usable ones like horses especially for mounts.