In "A Certain Scientific Railgun", how many "sisters" did Accelerator have to kill to achieve the rumored level 6?

In the certain scientific railgun series, Accelerator had to kill 20,000 sisters to achieve the rumored Level 6. The reason for this is that Accelerator possesses the ability to manipulate vectors, making him one of the most powerful espers in the series. As part of a cruel experiment called the "Sisters Project," the scientists created clones known as the Sisters who shared the same abilities as Accelerator. To achieve Level 6, Accelerator had to defeat and kill each of the 20,000 clones, pushing him to the extreme limits of his power and displaying the magnitude of his strength.