4 Mistral cores and two Monsoon cores.
Twitter user @never_released unearthed details about the A11 from Apple's recently leaked iOS 11 GM firmware; however, the original report mixed up the codenames and has since been corrected.
Correction: it's 4 little cores and 2 big ones, with all of them being able to run at the same time.
Developer Steve Troughton-Smith has also clarified:
Just to clarify on the A11: it's two high-power Monsoon cores and four low-power Mistral cores, all independently addressable. No Fusion
He also notes that "this sounds like it's going to be a Geekbench monster. RIP the rest of the industry."
The Apple A11's SoC number is T8015. The WiFi chip on the Apple A11 is a BCM4357, connected over PCIe.
Apple is expected to unveil its new iPhone X and A11 chip on September 12th. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.