7 billion (the current approximate world population) divided by 365 (days in the year) is about 19.2 million; but it doesn't necessarily help you to find out how many people are born, or could be born, on one day.
The world population is growing all the time and 7 billion is an approximate count of people alive today, many of whom were born decades ago when the population was lower.
You also need to take into account the fact that a woman has a limited period of about 30 years in which to bear a child and the gestation period is 40 weeks. So you may need to take into account how many women in the world are between, say, 15 and 45 out of the whole population.
It would be interesting to search for actual figures for daily births worldwide and see if you can come up with something of a formula.