As of now (2019), the Rank that they will retire would be PCIT i.e Principal Commissioner of Income Tax. The other posts above this are Chief Commissioner, Principal Chief Commissioner and finally Member, CBDT.
This may change if some cadre restructuring happens. Also, the batch sizes of the current incoming batches are pretty big. So all bets are off and you will have to wait and watch.
You can do a simple search by Batch Year and Rank here1 and can get to know how many people are there in a particular rank for that input.