Estimates vary greatly. It appears to be common knowledge (not to be confused with common sense or truth) that about one-in-ten people are gay (10% of the global population). Working on that basis, with an estimated world population of 7,074,819,990 (~7 billion; estimate January 2012) about 707,481,999 people (707 million) are gay.
In reality, it’s very difficult to know. It’s always going to be a guess based on asking populations questions about their lives, and trusting that they always tell the truth. Perhaps we can safely say that a good many more of the world have “experimented” but not continued, or that thanks to the persistence of several inhuman regimes, a great many also live in fear for their lives should their sexuality be exposed. It’s not so easy for those living in more liberal societies either- ultimately, true numbers cannot realistically be known.