How many lesbians are there?
This is pretty much impossible to answer accurately.
Not everyone will admit to being lesbian (or gay), even if they are. Or they may not acknowledge, even to themselves, that they are. And even those who are and admit it may not say so in a survey because they don't think it's anyone else's business. In some places, this number would be larger where social pressure was minimal, and much much smaller where social pressure was massive. In other words, people in a very strict social area would be very reluctant to answer truthfully and your numbers would be highly skewed. But even in more progressive places, there will still be a huge number of people unwilling to admit to being gay or lesbian so you will never get a real answer.
But we can get a rough guestimate if we go by the generally accepted 5 - 7% figure for percentage of gays. So if we presume there are 6 billion people on the planet (give or take a few million) and that about 50% are female, and of that 5% are lesbian, we get a very rough number of ~150 million.
But this is just a guess; we don't even know how accurate the 5 - 7% expectation really is, for all of the above reasons. Some people claim it's 15%, others say it's not even 1%, depending a great deal on their political agenda.