How could I tweak the Data Validation below which in cell J10 where it
resides, also include an ability to enter the letters CC?

At the moment it requires up to a 6 digit numeric value which must be
unique from the inputs in J10:J22. I still want to retain this but
also allow the letters CC. Note here I want to allow CC to be typed in
more than one cell in the range J10:J22 (but the up to 6 digit
numerics must be unique)

=AND(J10=1,J10<999999,COUNTIF($J10:J22,J10)=1)