Okay so I promised I'd discuss the nature of orc reproduction today and WHY they are in fact one of Morgoths' most lethal creations.
We know orcs breed fast. I admit this is stated by dwarfs and elves who are the slowest breeding things in the universe. (Arwen was 2800 years old before she got married.)
I'm speculating, but I think it's supported by the works. Imagine that orcs DON'T breed like humans. Instead they breed like dogs. A large dog is about about the size of an orc. They are pregnant about 80 days (same as a great dane). They give birth to a small litter of precocious children. Say 3-6 whelps, who are more or less abandoned to fend for themselves in a creche. A few guards get assigned to feed the orclings, and keep them from killing each other too much. Mom sticks around a few weeks as one of the guards, then leaves. If the orclings grow to adulthood at the rate of a dog, this means they can breed on their own in 2 years.
This isn't a weird preternatural ability. It's pretty common among mammals. A cow weighs more than 1000 lbs at 2 years of age. Mountain lions are 2 years old when they have their first litter. It's humans that are the outliers with our long gestation and growth. Why would orcs be similar? Why not be like other animals?
Morgoth's magic means orcs are born with a natural ability to learn Black Speech. If they're precocious the orcs can walk or crawl from birth. Think of lemurs - they are primates, but have motor skills early. Or goats & pigs - they can walk within a half hour of birth. This makes orcs more alien, which I think is good.
YES this also means that orcs, at age 2 and adult size, are going to be ABSOLUTELY CRAPPY warriors. Their skills will be limited. Sure they can stab you and figure out which end of a bow is the pointy bit, but most any human outfight a single orc. Don't worry. It's all part of the plan, and helps explain how the orcs get killed in such vast numbers in battles when they lose.
NOW FOR THE MATH. If orcs can breed at age 2, and each female has a litter of 3-6 orcs a year, then in the 78 years between the Battle of Five Armies and the War of the Ring, Sauron could have bred heaps of orcs. Assume the species was almost completely exterminated at Five Armies. Twenty years later there would be over 300,000 orcs.(assuming no losses). Thirty years after, almost 100 million orcs! By 78 years to the War of the Ring, there are theoretically 35 QUINTILLION orcs.
Obviously this isn't the case. They hunt, they fight, they die, they eat each other. I'm sure there are only a few million orcs by War of the Ring. But these creatures reproduce with amazing swiftness and that's what Sauron likes.
This is why Sauron doesn't care that orc warriors are inept. He can afford to lose 100 orcs for each human soldier he kills. He doesn't care when orcs kill each other at Cirith Ungol. Plenty more where THEY came from. And after all, orcs DO learn more skill in fighting over time, so eventually they're decent enough. Infighting just hones those skills.
Orcs don't need magic, super strength or anything else special to threaten Middle Earth. All they need is to be orcs. For leaders or elite troops, Sauron's got trolls, easterlings, and southrons. And a few remaining Black Numenoreans.
Why did Sauron breed Uruk-Hai? Surely they are slower to breed than orcs. I think the fact that Uruk-Hai can fight in the sunlight is a huge edge.
This means that an orc's family life is being raised in an underground warren, fighting for maggoty bread with your siblings, and being terrorized by bigger orcs. There are no orc babies in their cribs for Aragorn to massacre (sorry G. R. R. Martin). Just a terrifying unnatural species that destroys the world's natural balance. If allowed to grow unchecked, they will exterminate every other intelligent being.
You may argue that Sandy made a lot of assumptions here. And I did. But my assumptions match up with the movies and books AND more importantly make orcs a fearful force.