What is the limit of terminal ETS Leave Days?

Theoretically, the limit would be 95:
You can only carry over 60 days total to a new fiscal year.

30 SEP ETS example: 90 days total
If you carry over 60 on 1 OCT into your final FY in service, and your ETS is at the end of SEP that FY, meaning you will accrue another full 30 days before ETS, and you take no leave until you start transition leave, you would have up to 90 days for transition leave.

4 DEC ETS example: 95 days total
You carry over 60 on 1 OCT, you accrue another 30 that FY.
You take 30 days leave in SEP, carry 60 over on 1 OCT, take 60 days leave in OCT and NOV, accrue a total of 5 more during that, and take those 5 days leave 30 NOV-4 DEC.

Those are the limits unless you have accrued some kind of exempted leave that allows you to exceed carrying over 60 days to a new FY.