1. A men's 5 minute mile is not equivalent to a women's 6 minute mile.
For men a 5:00 is 77s off of the current WR, while for women, a 6:00 is 108s off of the current world record. A more similar mark would be a 5:30 for women.
2. Most people are saying 2-3% of the general women population(I'm guessing of the USA) could run sub 6:00. I would say the number is far less.
First, think of the age distributions in the USA, 20% of women are under age 14, and 45% are over age 35. I would imagine it is almost impossible to find women in those two age categories who could run under 6:00. That leaves only a fraction of the population(35%) between the ages of 15-34; which would be the most likely category to break 6:00. Of that 35% of the female population, according to government studies, 80% do NOT reach the weekly recommended amount of physical activity. Which would leave only about 7% of the population(age 15-34) who meet the national requirement of being an "active" person.
To meet the government's standards, it requires 60 minutes of exercise at least 3 days per week. Not necessarily a person who could break 6 minutes for the mile.
Just because one is deemed an "active" person by the government does not mean that they would then be able to run a sub 6:00 mile. Out of this 7% of "active" women, I would imagine only about 2/100 of them could then break 6:00 for the mile.
A conservative estimate would be approximately 0.14% of the population.