Yes, my broad advice for TM is simple: total storage of Mac or Macs to be backed up times at least about 3.
So if you have a Mac that has- say- 1TB of SSD and you shift a library or two OUTSIDE to- say- a 4TB drive, you have a 5TB Mac for this calculation.
Let's say you share a place with another person who has a Mac with another 1TB and they need sound backups too. Add their 1TB into the calculation.
5TB + 1TB = 6TB, so I'd buy TWO 18TB HDDs for TM backups, partitioning each as 15TB for Mac #1 and 3TB for Mac #2. Then set both up as TM for both Macs and store one of them OFFSITE after the backups complete. A good, cheap, secure offsite option is a bank safe deposit box but anywhere away from home/office that is safe & secure could work.
Regularly
rotate TM Drive A and Drive B so the offsite one is pretty up to date. For me, that's every 30 days or so. Worst case scenario for me is fire/flood/theft takes out both local Mac(s) and local TM drive on day 29 in that scenario. I could recover all from the offsite backup but I would lose the latest 29 days of additional files. If 30 days would be too much of a recent file loss for you, make your rotation day every 20 days or every 10 days or whatever works for you.
OR, come up with a little short-term strategy for added protection of latest files. For me, I regularly sync new files on a desktop with a laptop, so I end up with 2 copies of fresh files anyway (laptop usually goes out with me, so it is also offsite when I am). Others might store the newly created file in the Cloud so they are both sort of onsite
and offsite at the same time.
Why "times 3" instead of only a matching capacity amount? Because there are
TWO distinct advantages of TM:
- Mac Backup for recovery of everything in a worst-case scenario.
- The ability to go "back in time" to earlier versions of files. Sometimes, you can have a corrupted file and not realize it for a while. When you do realize it, you probably want to recover the last good version of the file. If you have a good amount of TM space, you can step backwards in time to the last good version and then recover it. A simple example of this benefit is imagining writing a book in an app like Pages. At some point, you accidentally delete chapters 2-6 but don't notice because you are regularly writing new stuff in chapter 18 to "the end." You finally write the final word to complete your masterpiece. Hooray! Now you want to review the whole book in a proof pass and that's when you discover the mistake. If you only have one "latest backup" of the book, you'll have to re-create chapters 2-6 from scratch. But if you can go back in time, you can go back to a version of the book file BEFORE the deletion and recover those chapters, reinsert them into the finished book and now you have the whole book!
Both of these benefits are important. And if the above storage examples were true, the difference in cost between a 6TB drive to match the capacity of the twin Macs vs. an 18TB drive per this 3X approach is only about $100 MAX as I write this, so $200 MAX for TWIN TM HDDs with 12TB of additional capacity to support that #2 benefit.
If $200 is too much, another "times 3X" approach is to base it upon existing total capacity used. So you check your 1TB Mac and discover you are only using about 300MB internally and maybe 1TB for that photo library on the externals and the other person is also using only about 300MB, you could multiple 300MB +1000MB + 300MB or 1.6TB times 3 = 4.8TB as minimum target TM drive size. Done this way, you might round that up to 5TB TM drives, which can be found for about $85 as I write this... or about $170 for two.
I prefer the full capacity approach because I assume data storage growth towards capacity over time. In 2 years, I'd rather the existing TM drives are still at least 3X actual capacity than fading to less than that because I've piled up more data, pictures, videos, etc. on my Macs.