When sociologists do research, they often come across a slight problem when gathering certain data. This isn't just exclusive to social sciences, of course. Although social sciences are known to be slightly harder to quantify than natural sciences. Let's take a topic such as sexuality. How can one quantify sexuality? How do you measure a person's sexual tendencies?
One American biologist named Alfred Kinsey wasn't afraid to ask this question, and most importantly, he wasn't afraid to do the scientific research needed to get the answer. He conducted research in regards to sex, the female orgasm, and sexual identity, among others. He was a firm believer that sexuality is fluid, but he still had to find a way to quantify it. As a result of his research, he developed the Kinsey scale.
The scale ranges from 0 to 6, where 0 means exclusively heterosexual and 6 means exclusively homosexual. Later on, a rating of X meaning "no socio-sexual contacts or reactions" was added. Where you are on the scale is determined by answering a series of questions about your experience in your lifetime.
On the Kinsey scale, bisexuality would fall under numbers two to five. This would also indicate that a lot of people have had at least a moment of homosexual intimacy, but don't identify as bisexual and simply see themselves as straight would fall under this category per Kinsey. But was does the data say?
As previously mentioned, these kinds of things can be very hard to quantify. There are a lot of things that need to be taken into account. For example, we don't have valid data that accounts for all the people in the world. There hasn't been a worldwide survey about the sexuality of people because there are still about 70 countries where homosexuality is a punishable crime. Sometimes, even punishable by death.
Most countries where we do have some data about sexuality are countries who are more liberal and have gay marriage or where LGBTQ have some amount of rights. For example, homosexual partnership is legal, but adoption and marriage isn't.
Another thing that should be taken into account is that there is still a lot of stigma. Even within the LGBTQ community, bisexual people still come across bisexual erasure. For those who don't know what bisexual erasure is, it's when people essentially behave like bisexuality isn't real or a "valid" sexuality. A person who is biphobic and therefore performing bisexual erasure might tell a bisexual person dating an opposite sex partner that they are straight all along, that they just say they like the same sex for attention, etc. Or they might tell the person that they are gay and scared to "fully come out". Bisexual erasure can take on many ugly forms, but these ways of expressing it seem to be the most popular. This leads to bisexual people feeling like they are not "good enough" for either group.
To answer the question "how many bisexual people are there", the best answer would be: probably much more than we think. Studies in America have shown that there is a steady rise in people identifying as bisexual, particularly amongst women.
With the data we have at hand, it's really hard to answer the question what percentage of women are bisexual. What we do know is that there is a trend in Western countries where people, especially young women, identify themselves as women. Surveys conducted in 2016 in Western cultures have gathered that 1% of the questioned population identifies as a bisexual woman.
However, times are changing, and they're changing fast. A more recent study amongst generations has shown that Generation Z is "the gayest age-group" we've had so far, with a whopping 9% of people who identify as bisexual. Five percent more than Millennials, six percent more than Generation X, and seven percent more than Boomers. Gen Z also has more people identifying as gay than any generation before. The survey also concluded that there is quite a disbalance between men who identify as gay versus women who identify as gay (a.k.a. lesbians).
To answer the question on a global scale, with some predictions at work, there is most likely about 1-2% of bisexual women on the world. How many bisexual women there are around you right now mostly depends on which country you are in. In most cases, there will be a higher percentage of lesbians if you live in a country where LGBTQ people are more accepted, but that doesn't have to be necessarily true. There will, however, be a big difference at times. For example, in 2016, a female-only study was conducted which concluded that 8% of Dutch women identify themselves as either gay or bisexual. In America, in a study conducted five years ago, 5.1% of women identified as LGBTQ.