Since women don’t die in childbirth as much as they used to, there’s no biological reason why they couldn’t.
One limitation is that breastfeeding may suppress ovulation, so mothers who breastfeed longer tend to space out their pregnancies a little more. But if the woman didn’t breastfeed, returned to having regular sex as soon as possible after childbirth, and stayed in general good health, there doesn’t seem to be any limitation to getting pregnant once a year, or even more often.