Which Cortex M to use to design face recognition application?

The Helium instruction set was developed to accelerate embedded ML applications, and so the Cortex-M55 or Cortex-M85 which implement this instruction set would generally be recommended. However it is possible to implement on many Cortex-M processors. See the below article, based on Cortex-M7:

https://developer.arm.com/documentation/102689

I like 3D face recognition

ARM cortex-M7 sounds good. www.nxp.com/.../nxp-enables-3d-face-recognition-for-secure-access-control-with-mcu-based-edgeready-solutions:NW-NXP-ENABLES-3D-FACE-RECOGNITION-FOR-SECURE