How much did the global population grow between 1950 and 2000?

According to an estimate provided by the United Nations, the world population in 1950 was 2.54 billion. The estimate for the year 2000 is more than 6.055 billion.
So the population increased by approximately 3.6 billion people.
These numbers represent a best guess based on data collected from a combination of sources, including government censuses, vital records (birth and death certificates), and surveys conducted by non-government organizations and academics. The sources are fallible, and in some countries, there is relatively little data collected. Thus, these estimates contain a degree of error.
For instance, Live Science has quoted an unnamed UN analyst as saying

"The uncertainty in census data is very high: in the range of 2 to 3 percent in most countries."

As the article author points out

"That range might not sound very large, but for a country like China, which has a population of 1 billion, that means a 40-to-60-million-person error."
https://www.livescience.com/15656-counting-world-population.html

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2781829/