In addition to the argument based on misreading a text as suggesting that Israel would be in Egypt for 430 years, there was also an argument made based on population growth. "How could it be that in only 210 years," we might paraphrase the question as stating, "the population of Jacob's family grew from 70 people to over two million?" The short answer is that the Israelites had large families.
For a more detailed answer, first of all, let's confirm that the numbers are correct:
Exodus 1:5 And all the souls that came out of the loins of Jacob were seventy souls: for Joseph was in Egypt already.
All the souls that came with Jacob into Egypt, which came out of his loins, besides Jacob's sons' wives, all the souls were threescore and six; and the sons of Joseph, which were born him in Egypt, were two souls: all the souls of the house of Jacob, which came into Egypt, were threescore and ten.
Specifically that 70 number is (breaking it down by their maternal connection to Jacob): 7 sons and grandsons of Bilhah (Genesis 46:25); 14 sons and grandsons of Rachel (Genesis 46:22); 16 (15 sons, grandsons, and great-grandsons and 1 daughter) offspring of Zilpah (Genesis 46:16); and 33 (32 sons, grandsons, and great-grandsons and 1 daughter) offspring of Leah (Genesis 46:15), for a grand total of 70 sons, daughters, grandsons, and great-grandsons of Jacob.
Additionally, as Genesis 46:26 informs us, there were wives that were not included in that number (nor, as we note above, was Israel himself included in that number). So, in essence, there were 69 men of Israel (Israel himself and 68 sons, grandsons, and great-grandsons, as well as two daughters) at the beginning of the 210 year period in Egypt.
Two years after the Exodus, there were 603,550 men (counting only those 20 years old and older) and not counting the Levites (Numbers 1:46-47). That's huge population growth in only 210 years. How did this happen? They had very large families. Scripture expresses it this way:
Exodus 1:7 And the children of Israel were fruitful, and increased abundantly, and multiplied, and waxed exceeding mighty; and the land was filled with them.
They didn't just have a lot of kids, they:
- were fruitful
- increased abundantly
- waxed exceeding mighty [numerically]
- land was filled with them
But again, after the initial wave of persecution (which the midwives resisted) Scripture tells us that the fecundity continued yet more:
Exodus 1:20 Therefore God dealt well with the midwives: and the people multiplied, and waxed very mighty.
There was an enormous boom in the population of Israel. How could this happen? The answer is simply that there were really enormous families. This is revealed to us in Scripture:
In Numbers 3:43, Moses and Aaron counted all the firstborn males of Israel - not counting the Levites. Now, keep in mind that the total number of males (over 20 years old) was 603,550. What Moses and Aaron found was that the firstborn males of the children of Israel from a month old and upward were 22,273 (Numbers 3:43).
That's a ratio of 1 firstborn male for every 27 males. Even if Israelites rarely had girls in those days (an unlikely proposition), that's an average family of 27 children. And actually, we've understated the matter, since the first-born males were counted from a month old and upward, but the total males were only counted from 20 years old and upward, and only those that were able to go forth to war (presumably this excludes any disabled or elderly men) (But see FN1, below). If we further assume that there were about an even number of girls born to boys born, then the average Hebrew family had over 50 children. These children may not have been born serially, since it possible that the Israelites were polygamists. If they were, their ever-increasing need for wives would tend to be met by taking Egyptian girls as the wives for their sons, something that would further have caused concern to the Egyptian Pharaoh, who would see it as decreasing his own population in each successive generation.
So, that's the answer to the objection to a 210 year stay in Egypt based on the massive population growth of the Israelites: they grew from 69 males to over 600,000 males (an 8,700-fold increase) by having enormous families. [FN2]
FN1: One further caveat is that the firstborn would tend to be the first to die of old age. Given that the 600,000 number counts those able to go to war, and excludes the elderly, the "old age effect" should roughly be canceled out.
FN2: For those interested in such things: 68 x 25 x 25 x 25 = 1,062,500, which shows how 68 men can, in three generations, become over 1,000,000: first generation multiplies 25-fold, second generation does the same, and third generation also does the same.