A lot of people in this thread are mixing up historical averages – with rampant disease, malnutrition, and poverty – with possible averages.
74 days is about 2.5 months. So having babies basically one year apart is quite doable for a lot of women.
Here is a list of anecdotes from women who got pregnant again within 6 weeks of giving birth.
Starting at age 20, as in the OP, probably wouldn’t get the average woman to 19 babies, but starting a few years earlier absolutely would.