There's something like 330 million Americans currently alive, give or take. Social Security began in 1935, so that's 89 years ago. For the sake of making the math easy for a dumb Lemmy comment, let's figure the population at the time was two thirds of what it is today at 220 million, and we can figure that within the margin of error virtually all of them are dead. Yes there are some Americans between the ages of 90 and 111 but they likely didn't have social security numbers as children; the practice of assigning a SSN at birth happened later when they tied it to a tax credit for having kids; at first you got a SSN when you got your first job so anyone who was under the age of 15 or so in 1935 wouldn't have been given one.

So let's figure 220 million Americans who have since died, and 330 Americans who are still alive, have held social security numbers. That's 550 million SSNs total. Rough back of the napkin math.