Accelerator was able to kil 10,000 clons of another level 5 esper, the 3rd most powerful esper. If he's so above other espers, why isnt he counted as a level 6 esper already? What does a esper need to count as level 6?
While they where clones of a level 5 esper, the experiment was a failure due to the clones being fairly weak:
The abilities of the Sisters were less than 1% as powerfufl as those of the original Railgun. On average they were merely Level 2, and even the strongest ones never developed beyond Level 3.
(Railgun S, episode 3)
If they could have cloned Misaka perfectly, they would only have needed 128 of her:
Based on the simulations, we have concluded that by preparing 128 kinds of battlefields, and killing the Railgun 128 times, the subject would shift to Level 6. However, as it proved impossible to procure multiple Railguns we have decided to use the Sisters, originally from the suspended Radio Noise projecct, as replacements. By deploying a large number of armed Sisters, and accounting for variance in the production process... By using 20,000 sisters in various combat scenarios, we may achieve a Level 6 Shift.
(Railgun S, episode 5, last line was typeset on the screen instead of being read aloud)
So Accelerator did not reach level 6 after killing 10,000 of the clones because they are so much weaker. (How exactly killing a million ants should make you be able to fight a bear makes no sense to me.) However I don't what counts as being level 6 or why they want to reach it (and how they planned to control Accelerator after the fact).