Simple math, cue the back of envelope. Let’s say the population is 7 billion, and average lifespan is 60 years. 60*365= 21,900 days (the heck with leap years). 7billion/21,900 = 319,634.7
Of course, in a lot of the world, (a) much of the population is very young; and (b) life expectancy can be more or less than 60 years. So allowing for (a) to outweigh (b) I’d guess about 250,000 die each day.
Adjust for the poulation being about 4 or 5 billion back in the 70’s and medicine being worse (shorter life expectancy) say the death rate was about 150,000 to 200,000 per day.
Working backward, 40,000*21,900 = 876 million. Wikipedia says the world population hit about 1 billion back in 1804, so I guess Blue Öyster Cult’s singing to their 1776 audience. Or, the number fitted the meter of the song…