From the review of your retention period, it won't be enough to figure out how many Tapes will be used.
Can you please confirm:
- Which Copy uses Tape ?
- Is it using Combine Source Stream or Multiplexing?
- Are you using Global Secondary Copy?
- Is 160TB the expected amount of Backed up Data?
- What LTO are the Tapes?
- Do we have any bad Tapes or Tapes with mount errors?
There are various criteria that you can use to work out an estimate (using raw capacity is easy enough), but it can be very tricky once you look into the above
In short there is no easy way, and if you want to drill down even further to get a number which is close to accurate for your organization, then the above consideration needs to be taken into account.
Hope that helps