While Tolkien's world of Middle-earth is expansive, if there's one thing it's lacking in, it's female characters. While there are a few important women throughout the Lord of the Rings series, they often take a back seat to the male characters. There are multiple instances of a race being mentioned only through its male members - like the Ents, for example, or the fact that only male Dwarves are ever seen in LOTR and The Hobbit (though female Dwarves do exist). Female Elves and Hobbits are more commonly discussed, as are female humans (in fact, Eowyn, one of the most important women in the series, is of the race of Men).
However, when it comes to the Orcs, women are essentially nowhere to be seen. While other races mention the females of their species - like how the Entwives went missing - the only Orcs that are ever seen are (presumably) male. This begs the question: do female Orcs even exist? If not, how do the Orcs procreate? Though these questions were not explicitly explored in the main story, there may still be answers for those who have scoured Tolkien's work and become experts on the subject.
Do Female Orcs Exist?
The question of where Orcs come from in the first place was never fully canonically answered, as Tolkien often changed his mind on the subject. The answer published in The Silmarillion is that Morgoth captured Elves and tortured them into Orcs in the First Age. However, this isn't even technically canonical because The Silmarillion was published from a collection of Tolkien's notes throughout different parts of his life, and Tolkien really went back and forth on the origins for Orcs, so a single decision on how they were created was never really made. In earlier tales of Middle-earth, Tolkien claimed that Orcs were created by Morgoth "from the heats and the slimes of the earth", though he later walked back on this when he decided that Evil could not create anything (creation could only come through Ilúvatar). Though he never came to a true conclusion about the origins of Orcs, it is established in The Hobbit that Orcs could reproduce, as Bolg is the son of Azog within the story.
This, of course, implies that there must be female Orcs if the race has the ability to procreate. Tolkien himself actually answered the question about Orc gender in one letter he wrote: "There must have been orc-women. But in stories that seldom if ever see the Orcs except as soldiers of armies in the service of the evil lords we naturally would not learn much about their lives. Not much was known." Amazon's recent foray into Middle-earth, The Rings of Power, actually did have some female Orcs on the show, but they look so much like the male Orcs that it was basically impossible to distinguish any sort of difference.
Why Are There No Female Orcs In Sauron's Army?
Tolkien's comment about female Orcs existing, just not in Sauron's armies, implies that there are Orcs that live outside the service of Evil. Unfortunately, it was never made clear exactly what he meant by this, as it's one of the questions he never really answered. However, his initial answer does confirm that female Orcs exist, so they have to be somewhere, right? If they're not in Sauron's army, where are they? Well, the answer may be that they are in Sauron's army after all, it's just that no one ever noticed. Unlike humans, the differences between the sexes might be barely noticeable, and it's entirely possible that there were female Orcs in Sauron's army the whole time.
In general, women aren't mentioned much over the course of the series, but they are there. This means that it's plausible that there were always female Orcs that just simply weren't noticed by the other characters. A lot of the characters aren't always the most observant, and they wouldn't know the subtle differences in the Orcs to be able to differentiate them. Since Tolkien said that Orcs were generally just seen as soldiers, then any female Orcs could have been soldiers as well. Much in the same way that female Dwarves aren't seen in The Lord of the Rings, but it is known that they still exist, the same could be true for Orc women. It's unfortunate that so little is known about female Orcs, as it's clearly a topic of interest for a lot of Tolkien fans, but it's one of the aspects of Tolkien's work that was never set in stone and is much more open for interpretation.