The answer depends on whether you are talking about "personal computing" or "business computing". It also depends on what data you have that is critical to have backed up.
To answer your question directly:
My backup HDs (I have two of them — my experience working in R & D and telecom large-scale software develoment
), are sitting on my desk. However…
I'm retired and do very little "professional work" any more, what little I do is mainly server-side programming. So that data (code, etc.) is backed up by the hosting provider AND I have a local copy on my personal computer (unlike many people, I don't trust some company to keep my data safe). The same goes for my personal websites where data also includes mySQL databases (I export any mySQL databases and download to my computer in addition to any backups my hosting company keeps).
Some data is "personally critical". For example, financial records and the huge number of passwords, URLs, user-ids, etc. The financial records are kept by my wife (her backup HD is on her desk), but there are two additional copies on USB memory sticks. One is kept in a fire-resistant lock-box in our bedroom (I forget how many hours it is guaranteed for — 5 or 8 I think). The other data I mentioned is managed with a "password vault" program. I use it on my smart phone ("phablet") and keep a backup on my computer (and hence the external backup drives) AND on Dropbox (I hate this "cloud" buzzword — it's on a server somewhere). The software does a backup ("sync") to Dropbox automatically whenever any entries are changed (edited, deleted, added).
What I used to do, many years ago when I was working and living in another city, is backup to two sets of disk cartridges (ioMega 250 MB Zip drives). I vaguely remember that it took 2 or 3 of them per backup (I haven't even calculated how many 100's it would take me now!). I kept one set on a bookshelf and dropped the other set at my sister's.
The basic rule that should be followed is, if it matters, then at least one full backup should be kept "off-site". People can figure out the reasons easily enough.