If you have a society where everybody is equally capable of bearing childen (lesbian separatist utopia, mpreg verse, Imperial Radch, etc.) then you only need 1-and-a-bit children per person who can bear children (and the and-a-bit is only there as a fudge factor; if you've got really low mortality you don't even need that.)
Basically, the replacement rate is actually 1:1 in pretty much every population: everybody dies exactly once, so everybody needs 1 replacement. With a high-youth-mortality rate, some people are dying before they can create their own replacement, so the survivors need to cover their replacements too, thus: lots of siblings for everyone.
(If you change the rule that everybody dies exactly once, the 1:1 ratio changes, but what happens to population dynamics when you mess with mortality existing AT ALL is a whole nother rant. And something people screw up even more.)
Population demographers complicate things by counting the birthrate as per "woman" (that is, adult-person-hypothetically-capable-of-bearing-children) rather than per person, which simplifies the stats-gathering in some ways, and also presumably stops people feeling funny about talking about men birthing. But that means you have to double your replacement number since you're only counting half the population as your base. (The slightly-more-than-2 rule is a fudge factor for messy reality, like the fact that the sex binary is not actually binary, and because they usually don't include girls who died in childhood in the averages for practical reasons; in hypothetical worldbuilding you can mostly ignore it.)
If you mess with the 'one man one woman required to make a baby' rule, the replacement birthrate becomes the inverse of the proportion of people who can birth children to the total population.
Which sounds complicated but really isn't: if 1/2 of your population can birth children, it's 2-children-per-birthing-parent. If 1/1 of your people can birth children, even if it requires six genetic donors to get them pregnant, it's 1-child-per-birthing-parent. If only 1/1000 people are queen mothers, they each have to have 1000 children. If you have three genders and two of them can birth children, but the gender proportion is 2:2:1, then 3/5 of your population can birth children, and the replacement rate per childbearing person is 5/3.
If you are in an a/b/o verse and have six sexes of which four can bear children, and all six are equally common in the population, then the average number of children per person who can bear children needs to be at least 6/4, or 2/3. But you can't just then say that the average number of children per couple should be 2/3, because some of your couples will have two child-bearing-capable parents in them, and that brings the overall average kids per couple back to 2.
So in a society organized around subunits of adults who are raising children - i.e., some parents and all their kids, where the vast majority of adults are parenting as part of a nuclear family - the shortcut is that for a stable population, the average number of kids being raised by a family should be the same as the number of adults parenting them, regardless of which, if any, of the adults were genetic or wombspace contributors.
So in your lesbian separatist parthenogetic society, the replacement birthrate is one child per woman, but if most women are living in couples, for a stable population, most couples should have two kids. And if most women are living in polyamorous communal farms of about a dozen parent-age women, there should be about a dozen kids in them too.
(If lots of adults aren't part of the childraising structure - say, your lesbian separatists aren't parthenogenetic, they just exile all the boychildren into the wildlands and then sex up the wildmen once a year at the festival - then your families-with-kids need to have extra kids to be replacements for the men out in the wildlands as well, so a non-parthenogenetic lesbian separatist commune of 12 parental-generation women should have about 12 girlchildren for their replacements, *and* twelve boychildren which they eventually exile.)
no subject
Basically, the replacement rate is actually 1:1 in pretty much every population: everybody dies exactly once, so everybody needs 1 replacement. With a high-youth-mortality rate, some people are dying before they can create their own replacement, so the survivors need to cover their replacements too, thus: lots of siblings for everyone.
(If you change the rule that everybody dies exactly once, the 1:1 ratio changes, but what happens to population dynamics when you mess with mortality existing AT ALL is a whole nother rant. And something people screw up even more.)
Population demographers complicate things by counting the birthrate as per "woman" (that is, adult-person-hypothetically-capable-of-bearing-children) rather than per person, which simplifies the stats-gathering in some ways, and also presumably stops people feeling funny about talking about men birthing. But that means you have to double your replacement number since you're only counting half the population as your base. (The slightly-more-than-2 rule is a fudge factor for messy reality, like the fact that the sex binary is not actually binary, and because they usually don't include girls who died in childhood in the averages for practical reasons; in hypothetical worldbuilding you can mostly ignore it.)
If you mess with the 'one man one woman required to make a baby' rule, the replacement birthrate becomes the inverse of the proportion of people who can birth children to the total population.
Which sounds complicated but really isn't: if 1/2 of your population can birth children, it's 2-children-per-birthing-parent. If 1/1 of your people can birth children, even if it requires six genetic donors to get them pregnant, it's 1-child-per-birthing-parent. If only 1/1000 people are queen mothers, they each have to have 1000 children. If you have three genders and two of them can birth children, but the gender proportion is 2:2:1, then 3/5 of your population can birth children, and the replacement rate per childbearing person is 5/3.
If you are in an a/b/o verse and have six sexes of which four can bear children, and all six are equally common in the population, then the average number of children per person who can bear children needs to be at least 6/4, or 2/3. But you can't just then say that the average number of children per couple should be 2/3, because some of your couples will have two child-bearing-capable parents in them, and that brings the overall average kids per couple back to 2.
So in a society organized around subunits of adults who are raising children - i.e., some parents and all their kids, where the vast majority of adults are parenting as part of a nuclear family - the shortcut is that for a stable population, the average number of kids being raised by a family should be the same as the number of adults parenting them, regardless of which, if any, of the adults were genetic or wombspace contributors.
So in your lesbian separatist parthenogetic society, the replacement birthrate is one child per woman, but if most women are living in couples, for a stable population, most couples should have two kids. And if most women are living in polyamorous communal farms of about a dozen parent-age women, there should be about a dozen kids in them too.
(If lots of adults aren't part of the childraising structure - say, your lesbian separatists aren't parthenogenetic, they just exile all the boychildren into the wildlands and then sex up the wildmen once a year at the festival - then your families-with-kids need to have extra kids to be replacements for the men out in the wildlands as well, so a non-parthenogenetic lesbian separatist commune of 12 parental-generation women should have about 12 girlchildren for their replacements, *and* twelve boychildren which they eventually exile.)