An agnostic is a person who doubts that God exists. They may believe that the existence of the supernatural is unknowable. When it comes to religion, they are usually characterized as having neither a belief nor non-belief in a deity (CIA Factbook). This is contrasted with an atheist who does not believe (sometimes strongly) that any type of deity exists in the universe.
According to some online dictionaries, the English biologist Thomas H. Huxley, who was a vocal advocate of Charles Darwin's theory of evolution, coined the word agnostic in 1869 A.D. He created it by combining the Greek prefix "a," which implies negative, with the word "gnostic," which means knowledge (Encyclopedia of World Religions, Cults and the Occult). Huxley stated his definition of the word was, "It (agnostic) simply means that a man shall not say he knows or believes that which he has no scientific grounds for professing to know or believe."
The word agnostic, however, is not a modern creation in spite of credit given to Huxley (Word Pictures in the New Testament)! The word is actually recorded by Luke when he wrote the book of Acts.
The Apostle Paul, in Athens, was struck by the city's public worship of countless gods. The people's fear of leaving out a deity worthy of worship, which risked receiving the wrath of such an offended god, moved them to create a unique altar. Paul, seeing the city's unique altar, was stirred to preach the gospel on Mars Hill.
For as I was passing through and observing the objects of your veneration, I also found an altar on which was inscribed, 'To an unknown (agnostos) God.' So then, He Whom you worship in ignorance is the One that I proclaim to you (Acts 17:23, HBFV).
The Greek word agnostos (Strong's #G57) used in Acts 17:23 is composed of an "a" that implies a negative (#G1) and "gnostos" (#G1110), which means known (having knowledge). The word, translated as "unknown," is where we get "agnostic," a person with no knowledge of God's existence.
How many are there?
A report based on a 2008 American religious identification survey found that 15% of the United States' adult population identify as having no religion. This set of people includes those who are agnostic, atheists or theists (belief in one or more gods). Unsurprisingly, 61% of this group believes in human evolution compared to 38% of America's general population (American Nones: The Profile of the No Religion Population).
According to a Pew Research Center 2012 global study, 16.3% of the world's population is not affiliated with a religion while 83.7% are (The Global Religious Landscape). The study also noted that many of the unaffiliated, which include those who are agnostic or atheists, still have various religious beliefs and practices.
In their 2017 article, sociologists Ariela Keysar and Juhem Navarro-Rivera, who reviewed many global studies on atheism, stated that there were 450 to 500 million positive atheists and agnostics worldwide. This would represent 5.8 to 6.5% of the world's 2019 population of 7.67 billion. China possesses the most atheists in the world with 200 million (A World of Atheism: Global Demographics).
Based on proof?
Interestingly, a study on global religiosity and secularism found that it is unlikely that most who identify as an agnostic or atheist base their decision to doubt (or not believe) in a deity on a careful analysis of scientific arguments. Most people, according to the study, do not really care about scientific rationalism (the belief that reason alone should be man's supreme authority). This explains why 75% of Americans believe in the paranormal (The Evolution of Popular Religiosity and Secularism by Gregory Paul).