Here on Earth or in the afterlife?
For the FLDS, @Broomstick has answered it, but see FLDS Lost Boys. A number of young males get kicked out to reduce competition.
For Mormons in the 19th century, some wound up without spouses, many others married second wives after the polygamous man died.
They would be married “for time”, only here and not in the afterlife instead of “for time and all eternity.”
Some may have left Utah, but I don’t know that much about that.
For the afterlife, when I was growing up, it wasn’t ever talked about, or at least I don’t remember.
We were strongly discouraged from asking too many questions or even thinking about theology too much. Accept what they say and don’t go down any rabbit holes.
Historically, the initial doctrine was that only a small percentage of people would get into the highest level of the Celestial Kingdom so it wasn’t an issue.
Joseph Smith’s theology tended to be that there was an elite who would have special powers, privileges or authority. He would then allow more people to have the privilege and then it would get spread out to more people, then the general population.
During Smith life, it was first that only he was allowed to have polygamy, then a few close lieutenants, then more of the senior leadership, then the next rank and then more of the general population.
The origin of Mormon polygamy is not completely clear, despite many historians studying it.
The first plural wife is likely to be Fanny Algers, a servant in the Smith household, and likely in the mid 1830s, which Fanny would have been 19 to 20, while Smith was 12 years her senior.
Oliver Cowdary, the second most important figure in Mormon, called it “dirty, nasty, filthy affair,” and this was one reason for him leaving the Church.
According to the LDS website:
Joseph married many additional wives and authorized other Latter-day Saints to practice plural marriage. The practice spread slowly at first. By June 1844, when Joseph died, approximately 29 men and 50 women had entered into plural marriage, in addition to Joseph and his wives. When the Saints entered the Salt Lake Valley in 1847, at least 196 men and 521 women had entered into plural marriages.20 Participants in these early plural marriages pledged to keep their involvement confidential, though they anticipated a time when the practice would be publicly acknowledged.
Nevertheless, rumors spread. A few men unscrupulously used these rumors to seduce women to join them in an unauthorized practice sometimes referred to as “spiritual wifery.”
I’ve read that 20% to 30% of Mormons were in polygamous families at the time, so the number of men would be smaller than that.
PBS says": 1894: The Church of the Latter-day Saints has 201,047 members.
Several of the influential apostles taught this in the second half of the 1800s. However, Brigham Young was the president and never taught this directly himself. Young taught that only people in polygamous marriages would become gods and goddesses, but he personally never taught a number.
The teachings were all over the place in the 1850s to 1880s. Some apostles were teaching that Jesus had three wives, but you can’t really point to any one quote and say this was the official doctrine.
However, I think it was universally taught that polygamy was a requirement for exaltation.
Walking a tightrope is never fun.
About nine or ten years ago, someone in the church headquarters leaked a presentation made to the top leadership discussing threats to the Church.
At the time, there was an Ordain Women movement which was attracting liberal Mormons. John Dehlin has a podcast / Youtube channel and draws a lot of liberal Mormons as well as ex-Mormons. I used to listen until I got my fill of the subject.
The blue are the sins which the Church believes that draws people away from the Gospel.
On the right, in green are people who have made claims that the LDS church as lost it’s way. This graphic is dated, but this popped in the Lori Vallow Daybell. , this was kind of respresentive on the movement, although fortunately, most of these groups don’t believe in killing people.
I have a close relative who was approached to be a plural wife. My aunt and uncle were in a study group and some polygamous people heard about the group and joined to get recruits.