Hello Ken6503,

You haven't shown us the expected output, so I am not sure either we need to put both conditions of having number of fields lesser than 5 + $NF(last field's) value should be grater than 2. But by seeing your expression I have made following.

Code:

 awk -F, '{if(NF<5 && $NF+0 > 2){$2="XXX" OFS $2;print;next}} 1' OFS=,  Input_file

Output will be as follows.

Code:

aaa,bbb,ccc,fff
aaa,bbb,ccc,ddd
111,XXX,222,333,444
111,XXX,222,333,444
111,222,333,444,555

Thanks,
R. Singh