**How do you define the biological limit?**

It very much depends on how you define a biological limit. If you prefer, it very much depends on how you define a human!

There is genetic variance underlying fecundity that is important to consider. For example, in your calculation you took lifespan as a constant but it obviously is not a constant. Fecundity is reduced by all these small mutations segregating in the human populations that reduce the fecundity of any single human. There exist no human that is free from all deleterious alleles.

*The definition I am using to answer this question*

In the below answer, I will assume that by biological limit you mean the mean fitness of a female that would live in the same environment than we live in today but would not carry any single of the deleterious locus that are segregating in the population. In other words I assume that you accept that the "perfect woman", that is a woman that would not carry any single of the deleterious mutations that segregate in the populations will set this biological limit to women fertility.

**Mutation load Theory**

The decrease in the mean fitness of an idealized population where no deleterious mutations would exist compared to the realized mean fitness of a population is called the mutation load. The mutation (in an infinite population) is a function of the genome-wide mutation rate only. Let the genome-wide mutation rate in human be $U=2.2$, the mutation load $L$ in the human population is equal to $L=1-e^{-2.2}=0.89$. The estimates are taken from [this video][1]. You may want to have a look to the video in order to have a better understanding of why $L=1-e^{-U}$ and what are the assumptions of these calculations (infinite population size, multiplicity of fitness effects and additivity of fitness effect are the three main assumptions).

**So what does mutation load theory predicts?**

The above calculation means that segregating deleterious allele reduce the fitness of the average woman by 89%! If we assume that an average woman can have 10 babies (this number is just a random guess) if she wants to, then the maximum biological limit of a "perfect woman" (in a world where there is no more competition than the one in which we are living) is $\frac{10}{1-0.89}≈91$  babies!

According to [wikipedia][2], the world record is 69 and is owned by Mrs. Vassilyev. It is reinsuring that the above calculations make sense because 69 is lower than 91. If we were to know the distribution of maximal number of babies per woman (not to be confused with the realized number of babies), we could calculate the probability to have 69 or more babies and compare this probability to the total number of females that has ever lived (or the total number of females that would have never had the idea to have so many babies). Always according to wikipedia, the world record for men would be 867 and is owned by [Ismail Ibn Sharif][3]. However, sounds possible to me (but I really have no idea) that some sperm donor may have overshoot this number today.


  [1]: https://www.youtube.com/watch?v=jctvV5xrWw4&spfreload=10
  [2]: http://en.wikipedia.org/wiki/List_of_people_with_the_most_children
  [3]: http://en.wikipedia.org/wiki/Ismail_Ibn_Sharif