In ballet what percentage of people actually end up being a principal dancer?
There are no reliable sources of statistics on the percentage of ballet students who go on to become principal dancers- of either gender. Ballet is a highly competitive art and it is safe to say that only people with dedication to training, passion for the art, and natural talent become principal dancers. Hundreds of students of a variety of ages may be part of a ballet school (or dance academy) at one time. Those who are on the training track for professional dancing- often called a pre-professional program- can be upwards of one hundred per academy. This is dependent on the size of the school and number of teachers available. Pre-professional programs are quite selective though, so their numbers are typically small.
You may know that ballet companies are even more selective and may not have a large number of dancers in the company. The highest ranking dancers in the company are called the principal dancers. The New York City Ballet has 93 dancers of varying ranks, not including students who may be chosen to perform specific roles. Of the 93, only 23 are principals. That's only about 25% of the total company. So, if someone is hired to dance as part of a company, we could assume they have about a 25% chance of becoming a principal.
However, making it through auditions for a ballet company is very challenging. Out of the hundreds of students who study with the NYCB in any given year, only a very small number of them will go on to be principals. I would put the odds at maybe 1% or less. I attended a ballet school and worked with their administrative department. Maybe two or three of the highest ranking dancers in the company had grown up in the academy, which had around three hundred students at any time.