What is to rockwell hardness of 420 stainless steel?

ANSWER: Rockwell (HRC) 50.
Grade 420 stainless steel (Fe, <0.15% C, 12.0-14.0% Cr, <1.0% Mn, <1.0% Si, <0.04% P, >0.03% S) is a higher carbon version of 410; like most non-stainless steels it can be hardened by heat treatment. It contains a minimum of 12 per cent chromium, just sufficient to give corrosion resistance properties. It has good ductility in the annealed condition but is capable of being hardened up to Rockwell Hardness 50HRC, the highest hardness of the 12 per cent chromium grades.
An exception to this might be the new Carpenter CTS alloys which have recently been released. They can achieve higher Rockwell HRC stylus tests.

(Some knife makers will claim higher HRC hardness levels from *THEIR 420 SS [such as HRC58], but this is [as of now], impossible.)