The 10% figure comes from (*drumroll*) Alfred Kinsey... Or rather, it originated in his book, Sexual Behavior in the Human Male.
Does this mean that 10% of the population is homosexual? No, it means that, of the 5300 white males polled by Kinsey and his peers, 10% answered that they were predominately homosexual. That's all.
Different polls and different sources have their own figures. For example, a 1993 Janus Report estimated that 9% of men, and 5% of women, had more than "occasional" homosexual relationships. While The Family Research Report says "around 2-3% of men, and 2% of women, are homosexual or bisexual." And The National Gay and Lesbian Task Force estimates 3-8% of both sexes.
One of the biggest issues comes from agreeing on a definition. As someone pointed out in the comments, it's hard to establish exactly what "homosexual" actually means. For example, if a man lives and dies, and had one homosexual experienced during his lifetime: Was he gay, bisexual or straight? Whatever category you'd place that man into, Kinsey's reports indicate that roughly 37% of men would fall into it. So it all depends how you classify sexuality.
And to make matters even more confusing, some (like the Canadia based Centre for Addiction and Mental Health) suggest "sexual orientation may be fluid and change over time". So even if someone identifies with being straight at one point in their lives, at another point they may naturally find themselves identifying with homosexuality.
In short: There's no definite, conclusive answer.