1557 illustration of a woman pregnant with 11 foetuses, supporting the weight with a large girdle. According to the accompanying story, this is her second pregnancy; her first was of nine.

From Prodigiorum ac ostentorum chronicon by Conrad Lycosthenes.

Now, it's very tempting to immediately call bullshit on this story, as pregnancies with a very high number of foetuses are often linked to fertility treatment, which didn't exist in the 15th century when Giovanni Pico della Mirandola told the story.
It becomes even more tempting to dismiss the story out of hand when you look at other things which turned up in the same book, which included things such as jaunty baby with abdominal heart, concerned torso dude, Chicken Legs McLionman, and whatever the fuck is happening here.
However, we'll bite. No question is too small for the Vagina Museum. Could a woman in the 15th century give birth to 20 babies across two pregnancies?
First of all, we'll turn back to Prodigiorum ac ostentorum chronicon. As well as reporting some entirely bizarre stuff, it also recorded more normal wonders from around the world. Such as Halley's Comet. Or moose. Or rhinos, which it admittedly embellished with a fancy costume.

So we can't just dismiss Lycosthenes entirely as a premium-grade bullshitter.

Now, we'll say again, multiple pregnancies with a high number of foetuses are incredibly rare. The chance of having quadruplets is about 1 in 600,000. The chances of having sextuplets rises to 1 in 4 billion. And the highest verified multiple pregnancy has been nonuplets - nine babies in total. This is not looking good for Lycosthenes.

To make a multiple pregnancy, you need multiple eggs (with the exception of identical twins). This is why high-number multiple pregnancies are usually linked to fertility treatment: either implantation of multiple embryos, or drugs to stimulate ovulation for egg harvest.
Present-day fertility treatments did NOT exist in the 15th century, so this would not have caused a pregnancy of 11 foetuses. Nevertheless, multiple ovulations can and do happen naturally sometimes. In fact, the detail that the woman's previous pregnancy had also been a multiple suggests maybe the story could be true: multiple ovulation is linked to genetics (which is also why non-identical twins sometimes "run in families")
The human uterus is a very stretchy organ. However, its capacity is finite, and further limited by all the other internal organs and the human pelvis. This means multiple pregnancies are very risky in the present day, and were even riskier in the past. That the woman's doctor is not named despite having seen a woman through such incredible multiple births further suggests the story is entirely made up.
The story doesn't say whether any surviving babies were born from either pregnancy. If it happened, it's very unlikely. Preterm birth is a common risk of multiple pregnancy, even with twins. Again, you'd think they'd have included this detail in the story if it were true.

The last time the story appeared in a medical text was in the 19th century book "Anomolies and Curiosities in Medicine" by Gould and Pyle. Do you want to know what they said about it?

"Reputed births in excess of 6 must be looked on with suspicion, and, in fact, in the great majority of reports are apochryphal."

Yeah, they called bullshit on it, too.

So, to conclude, it's pretty implausible that a woman in the 15th century could survive a pregnancy of 9 to have a pregnancy of 11 subsequently. But it is a bit more plausible than a man with dogs for knees.
@vagina_museum the drawing is also just extremely hilarious and relatable in a way lol