I also have 12. So decided to duplicate the test for another data point:
With 1 super human assigned to Endless Knowledge I gained 1B skill points in 17.40s
With 12 super humans assigned to EK I gained 1B skill points in 17.41s (Honestly I was impressed at my precision here lmao)
So yeah I would agree with @docnutnut that anything beyond the first does nothing at all.