How much does world population increase each day?

This number changes from day to day, and year to year. The best estimates use long term forecasts for the most accurate average such as growth per decade.

Anyway, if you base the rate purely on, say a 2008 population growth rate:

The world population is growing at a rate of 1.16% (78 million people) per year in 2008 (compared to the year 2000 as a baseline), or approximately 212,970 people per day.