1. The uterus is designed to grow to at least 500-1,000x its normal size and roughly 16x its normal weight, plus holding about 9-15 pounds of baby, fluid, placenta, etc. with a singleton pregnancy. Hit the escalators if it's multiples. If Octomom's uterus didn't explode, I think it's safe to say that there's no "limit".
2. The uterus is designed to do all these things over 8-9 months. The body can more easily adapt to changes over long periods of time. If you tried to expand the uterus that much in a span of minutes or hours or days, it probably wouldn't go so well.

(March 11, 2020, 9:51 am)doubleintegral 1.  The uterus is designed to grow to at least 500-1,000x its normal size and roughly 16x its normal weight, plus holding about 9-15 pounds of baby, fluid, placenta, etc. with a singleton pregnancy.  Hit the escalators if it's multiples.  If Octomom's uterus didn't explode, I think it's safe to say that there's no "limit".
2.  The uterus is designed to do all these things over 8-9 months.  The body can more easily adapt to changes over long periods of time.  If you tried to expand the uterus that much in a span of minutes or hours or days, it probably wouldn't go so well.

The uterus is a fascinating and beautiful thing. It appears that nature designed it very well. It is virtually flawless in its function which is to provide a safe, protective and nurturing environment for the fetus to grow and develop. I can't imagine what it must be like to know you have something that awesome inside you.

Human tissue can grow. So I guess no limit. Just don't grow too fast. But the mother will need to constantly feed the babies depending on how many. I've seen a starved pregnant cat misscarriage due to not being fed over a period of time.