I'm sure they had breeding programs and measures to make sure that the Hork-Bajir wouldn't go extinct, but they were too careless about them. Doing genetic experiments, casul executions (Visser 3 at one point throws one out an air lock because it broke his concentration) and we know that the Yeerks used Hork-Bajir and . . . well, we don't know of anything else, as their front line soldiers and as garrison soldiers, Taxxons are implied as being more numberous yet still are more like a second line soldier so on a galactic scale the Yeerks were probably winning some fights, but any time they lost a battle, or lost a pool ship how many Hork-Bajir did they lose, and could they replace them quickly enough?
I remember now that they mention Hork-Bajir reach maturity fast, Toby was an adult by the end of the series wasn't she? So a 2-3 earth year wait for a combat ready Hork-Bajir controller . . . I mean it's possible that the Yeerks could maintain Hork-Bajir numbers during the war, certainly better than the Andalites, when but I still feel like with carelessness like that shown by Visser 3 they would have seen numbers dropping gradually, or even rapidly.
I mean in the end what did they plan to use humans for? They don't pilot bug fighters, they don't go into combat, they might be technicians and they do help to crew the Blade Ship and Pool Ship, I guess freeing Hork-Bajir and Taxxons up to be used in ground combat, but still did Visser 3 have a quota of humans needed, a number he had to maintain thus explaining why he didn't risk them very often? I mean why play an ace if a two will do? Why risk a Hork-Bajir when you can use a human? He's on earth, he can replace humans much more easily, kidnap homeless people off the street, no one would notice or care, it doesnt matter that they're not voluntary or that they're not in a position of power, for a dracon toting grunt to be thrown at the andalite bandits you don't need the president of the united states, just a body with a trigger finger.
There is a Japanese saying that goes something like buy 100 spears for 10 gold, instead of one sword for 1,000 gold, losing even one Hork-Bajir would have been worse than losing a few humans, Visser 3 must have lost well over a hundred to the Animorphs even before the free Hork-Bajir started abducting them too, all in all I doubt his losses would have reached one thousand, but before becoming Visser 1 I doubt he should have had many more than a thousand of these soldiers on a simple harvest world, they'd need more of them for the worlds that do know they're being invaded and to fend off the Andalite fleets and armies.
The Animorphs went out of their way at times not to harm humans so losses in Yeerk life would have been reduced as well.
Instead Visser 3 says <Hork-Bajir . . . go use your blades, I want some entertainment . . . no, put the dracon beams down, I said use the blades.>
If I were a Hork-Bajir controller under his command I'd hop into a bug fighter and fly straight to Visser 4 with a copy of Visser 3's personal diary and say "Listen up dude, you want to be Visser 3, I want to make the current one look bad, give me a higher rank and you get to know who Visser 3 calls his "widdle schmoopykins"
Except it'd come out more like "Ghafrash, be Visser 3, ghafrash higher rank, fit me know, eked widdle schmoopykins!"
