My Mini 2 had a minor altercation with light weight outer branches of a tree today, the drone recoved and kept flying.  After the flight I noticed one propeller blade had 1 - 2 mm nicks at the end, the other blade on that motor looks fine.  Is it recommended to replace both blades even if the second one looks OK?  I note the replacement blades from DJI come 2 to a pack.

Yes, it is recommended to replace both.

However, I've replaced only one before with no issues.

Hello there FlyingOverABQ. I am sorry for the trouble this has caused and thank you for reaching out. DJI recommends that it would be best to replace both propellers of the said DJI Mini 2 for safety purposes. Thank you and fly safe always.

The props come in twos as they're supposed to be a matched pair, for dynamic stability. I'd change both, to be on the safe side.

Always swap in pairs... It even says so in the manual.

The props are apired for weight and any slight difference could cause additional stress on the motor and /or unstable flight.