How many Israelites were there when Jacob (AKA Israel) was born? Well, I don’t think he counts. Let’s define Israelites as descendants of Israel (AKA Jacob). So then the answer would be zero.
When Jacob’s family moved to Egypt when he was 130 years old, there were about 70 descendants of Israel. (The exact number given varies between different parts of the Bible. It could be anywhere from 66 to 75.)
One Pharaoh later, it says the Israelites multiplied greatly and filled the land so much that the Egyptians felt threatened. By the time of the exodus, they had about 600,000 men, plus women and children. So they went from less than a hundred to over a million people during the time they lived in Egypt, which the Bible says was no more than 430 years.
And how many generations did that take, to get from 70 to over a million? About two? Kohath was one of the 70 who moved to Egypt, Amram was his son, and Moses was his son. (Moses was 80 by the time the exodus happened, but if they were in Egypt 430 years then these generations would have to be like 175 years apart. So Moses’s age doesn’t make much difference. It’s still just two generations.)
For the population to grow that much in just two generations, each woman would have to have around 300 children. I suppose that could happen… if, say, they all kept getting pregnant nonstop from age 5 to age 73, and each pregnancy only lasted 21 weeks, and they all had twins every time, and no one died. Does that seem likely? At that rate, it would only take two more generations before there would be more Israelites than there are people in the world today. No wonder the Egyptians were nervous. But that wasn’t too much for just two midwives to handle, apparently.
Shortly after the exodus, Moses describes the Israelites as being as numerous as the stars in the sky. How many is that? Well, the number of stars in the observable universe is like a trillion times more than the number of humans who will ever exist. But the number of stars visible to the naked eye from Earth in the absence of light pollution is less than 50,000. I’m gonna say the second interpretation is way closer to a reasonable number of people. Plus, that’s what it sounds like the Bible meant the first time it compared people to stars. So… the number of Israelites suddenly fell from over a million to less than 50,000?
Then just a year after the exodus, Moses takes a census and counts over 600,000 non-Levite men at least 20 years old, and at least 22,000 Levite males at least a month old. So now they’re back over 1.2 million people. And that’s not counting most of the people under 20.
About 40 years later, Moses took another census. Almost all the Israelites from the first one had already died by this time, but the numbers weren’t much different. There were still over 600,000 Israelite men who were 20 or older, and 23,000 male Levites at least a month old.
When Moses died, not long after that, he said again that the Israelites were as numerous as the stars in the sky. So I guess they’re back below 50,000 again.
In the days of the judges, it says there were 25,600 fighting men in the tribe of Benjamin. Multiplying that by 12 tribes, it seems like there would have only been about 300,000 fighting men in all of Israel then, instead of 600,000. Except Benjamin was a particularly small tribe, so I guess there could have still been around the same total number of men there had been… At least until the other Israelites murdered most of the Benjamites, which would reduce the number of Israelites by more than 50,000.
Wait, is that why Benjamin was a small tribe? Maybe it is; maybe 25,000 men was typical for an Israelite tribe before that happened. In which case we can go with the estimate of 300,000 Israelite men. So the total number of Israelites would be over 600,000, but after the Benjamite genocide it would be less than 600,000, maybe. It’s hard to tell exactly, when the Bible keeps leaving so many people out of the count because they’re the wrong age, sex, tribe, etc. for fighting.
When Saul became king, it says there were indeed only 300,000 fighting men of Israel, plus 30,000 fighting men of Judah. (Who they’re counting separately for some reason, even though Judah was supposedly part of the kingdom of Israel at that time.) When David became king of all Israel, just 50 years later, Israel only had 30,000 fighting men. What happened?
Then during Absalom’s brief reign, it says the people of Israel “and Judah” were “as numerous as the sand on the seashore“. How numerous is that? Well, there are about 372,000 miles of coastline in the world, and about 31% of those are sandy. So that’s about 115,000 miles of sandy shores. The coastline of Israel is 170 miles, but ancient Israel had different borders. Their coastline was maybe 2/3 of that, which is around 115 miles.
So Israel’s seashore was about a thousandth of all the world’s beaches. The number of grains of sand on all the beaches in the world has been estimated to be in the quintillions. So the grains of sand on the seashore of ancient Israel would probably be in the quadrillions.
When God told David to take a census and then got mad that David took a census, there were either 1.1 million fighting men in Israel (including Judah), or 1.3 million in Israel “and” Judah, depending on which part of the Bible you believe. Either way, their total population would now be more than 2 million.
And then during Solomon’s reign, one part of the Bible says they were as numerous as the sand on the seashore again, which is a thoroughly unreasonable number of people. And another part says they were as numerous as the dust of the earth, which is several orders of magnitude worse. I’m making a graph of these wild population swings, but I’m going to have to leave out these sand and dust numbers. At that scale, you wouldn’t be able to see anything else.
After the kingdom splits in two a bit later, their total population is still over 2 million people. Israel has 800,000 fighting men, and Judah 400,000.
And by Jeremiah’s time, the population had apparently spiked to completely absurdly high levels again. So high that God was threatening to punish his people by making just their widows more numerous than the sand of the sea.
(I don’t know what the Hebrew population levels were like in real life back in BC times. But for the last 2000 years, they’ve generally been in the millions.)