how many babies CAN fit inside a pregnant woman?

There is the scientific answer:

...the largest reported number of fetuses in one womb was 15...The largest set of children to be born together and survive—not including this week's octuplets, who are all still alive—is seven.
How do so many babies fit in one woman? The limit isn't so much the number of babies as their volume and weight. In general, once the total weight of the babies inside reaches about 12 pounds, the uterus goes into labor. The greater the number of fetuses, the earlier the labor will occur. (The rough formula for due dates is to start at 40 weeks of gestation for one baby and subtract three weeks for each additional child: 37 for twins, 34 for triplets, etc. It's remarkable, then, that eight fetuses were able to gestate for 30 weeks, as in the California case.) ... If the uterus gets too big and the fetuses too heavy, it can be difficult for the woman to breathe. The amount of placental tissue can also cause the mother to develop pre-eclampsia, or pregnancy-related hypertension. seniors world chronicle

And the Mommy with a 34+ week baby inside her tummy answer: 1 is plenty!